As of recent I've been reading some Wikipedia articles on gender and sexuality leading out from the LGBT (lesbian, gay, bisexual, and transgender) page. However, it was not clear to me why the former term transgendered, which may or may not have been in so common usage, would have been dropped in favor of the term transgender, which seems like it sounds more like a noun than an adjective, unlike lesbian, gay, and bisexual, which to me sound more like adjectives or at the most adverbs than anything else, in what I would regard as proper English usage, correct me if I'm wrong here.
I am an actual transgender person, so perhaps I should speak to this issue.
It is overwhelmingly preferred among my peers that the term transgender be used and that transgendered be avoided. I am a transgender woman.
The verb is transition: I have transitioned. Transgender is part of my identity, as is Asian and bisexual. We do not say I have Asianed or bisexualed, nor does this usage have any bearing on irrelevant nature/nurture arguments. I have not always considered myself bisexual, but there was no process by which I was modified to become bisexual. It is simply a term that best describes my identity at the time.
We call this identity-first language, and it is used because it emphasizes the membership of a person within a specific community. This is important, because it is, in North American culture at least, relevant that a person is transgender. Being transgender has a tremendous impact on how I exist in the world.
Therefore, we appropriately use the term transgender as an adjective within the community in a convergence with other communities moving to identity-first language. Transgender is not a verb that makes sense, so transgendered is nonsensical. And a transgender is also considered to be offensive and dehumanizing, as this usage strips us of our personhood, because of the power dynamics and sociopolitical nature of identity.
Calling me "a transgender" would erase both my personhood and my womanhood; it instantly casts me as an other, an anomaly, which further underscores the very real struggles transgender people experience in society. In this regard, "gay" and "lesbian" as nouns are the exception, not the rule. If anything, conceptual proximity of gender identity to sexual orientation has led people to assume that "a transgender" may be acceptable or even correct. It is not.
People who are not transgender (the term for this is cisgender) are not materially affected by the language used for transgender people, but transgender people are very much affected by the language used to describe us. For this reason, it is important to use the language that people within the community specify.
Language is at the core of political and social struggles for rights and equality.
Word usages often have meanings that could affect political rhetoric. One of the key issues in gender and sexuality studies is the debate over the topic if sexuality, or certain sexuality, is innate--something people are born with. This debate has raged for decades. Pro LGBT rights activists would argue any kind of sexuality is inborn and intrinsic, countering a previous (20th century) view that homosexuality is a mental condition that can be fixed.
Same goes for transgender/transsexual. It is increasingly argued that transgender is something some people are born with and not an acquired inclination through culture. This topic is central to the debate of nature versus culture. As a tweet puts it
“Transgendered” is the linguistic equivalent of describing someone as “blacked.”
The origin of "-ed": Transgender as an act of will
The "-ed" suffix was originally put there because people incorrectly equated sex reassignment surgery with transgender people and mistook transgender as a process and an act of will. A lot of people confuse gender with sex, leading to transgender being confused with transsexual and transsexual surgical procedures. The modern consensus in gender and sexuality studies points to a distinction between gender and sex: whereas sex is a certain configuration of one's reproductive system as part of their anatomical structure, gender is differentiated from sex as one's personal identification or social role.
The genesis of transgendered lies in the confusion over gender and sex, transgender and transsexual. Thus people who support those distinctions argue against the usage of transgendered.
I have decided to include in my answer sources Andrew's answer already cites, because I think some of them should be supplemented with explanation. Echos of my explanation above can be found in these articles: Vox:
As trans advocate Joanne Herman noted in the Huffington Post, calling someone transgendered is a bit like calling someone "colored." "One problem with this label was that it implied something happened to make the person 'of color,' which denied the person's dignity of being born that way," Herman wrote. Similarly, transgendered suggests that being trans is something that happens to someone, as opposed to an identity someone is born with.
The implication behind transgendered flies in the face of science: people can know their gender identity at a very young age...
Transgendered is also unnecessarily long and confusing. LGBT group GLAAD explained: "The adjective transgender should never have an extraneous '-ed' tacked onto the end. An '-ed' suffix adds unnecessary length to the word and can cause tense confusion and grammatical errors. It also brings transgender into alignment with lesbian, gay, and bisexual. You would not say that Elton John is 'gayed' or Ellen DeGeneres is 'lesbianed,' therefore you would not say Chaz Bono is 'transgendered.'"
“The consensus now seems to be that transgender is better stylistically and grammatically,” DiEdoardo says. “In the same sense, I’m an Italian-American, not an Italianed-American.” The most common objection to the word, says Serano, is that the “ed” makes it sound like “something has been done to us,” as if they weren’t the same person all along. DiEdoardo illustrates this point, hilariously, with a faux voiceover: “One day John Jones was leading a normal, middle-class American life when suddenly he was zapped with a transgender ray!”
Moving away from the “ed”—which sounds like a past-tense, completed verb that marks a distinct time before and a time after— helps move away from some common misconceptions about what it means to be transgender.
One is that being transgender might be a choice that involves a person simply deciding to be that way or a result of something that happened to them, like sexual abuse. The majority of trans people I’ve spoken to have said they knew they had feelings of identifying as a boy (when assigned female) or girl (when assigned male) as far back as they can remember—even if they didn’t have the vocabulary or understanding to articulate what was going on—and even if they tried to change or stifle those feelings for half their lives. Imagine how it would sound if one described people as “gayed” or “femaled,” as if there was a point when that wasn’t the case.
Another misconception is that the defining part of being transgender is having surgery, as if a trans person isn’t really trans until they’ve gone under the knife and come out the other side fully “transgendered.”
Ngram would seem to support your observation (although Ngram data ends at 2008).
This may be simply a matter of opinion about the nuance of the "-ed" ending. There is this explanation from Vox:
As trans advocate Joanne Herman noted in the Huffington Post, calling someone transgendered is a bit like calling someone "colored." "One problem with this label was that it implied something happened to make the person 'of color,' which denied the person's dignity of being born that way," Herman wrote. Similarly, transgendered suggests that being trans is something that happens to someone, as opposed to an identity someone is born with. ("Why you should always use "transgender" instead of "transgendered"" - Vox.com)
And a similar opinion from Time magazine:
“The consensus now seems to be that transgender is better stylistically and grammatically,” DiEdoardo says. “In the same sense, I’m an Italian-American, not an Italianed-American.” The most common objection to the word, says Serano, is that the “ed” makes it sound like “something has been done to us,” as if they weren’t the same person all along. ("Why It's Best to Avoid the Word 'Transgendered'" - Time.com)
However, the Time article also points out that transgender is awkward as a noun, and should instead be used as an adjective, i.e. a "transgender person", which is the same as bisexual, gay, and lesbian.
In the end the evolving vocabulary of identity politics can be hard to follow. The best option is to repeat back what you hear from the people closest to the issue.
Transgendered is better. The "ed" ending is commonly used to turn nouns that are attributes, such as gender, into adjectives, such as gendered. Calling someone "transgender" instead is like calling someone "blue-eye" instead of blue-eyed. Of course "colored" is not the current preferred nomenclature, but when would you ever refer to a "color person?" Yes, nouns can be used as adjectives directly too, as in "I'm a dog person," but the meaning is different. "He's an ass man" means that he specifically appreciates asses, but "he's an assed man" means that he's a man in possession of an ass. Similarly, "He's a transgendered man" means that he's a man in possession of a gender, specifically a trans gender as opposed to a cis gender.