Is it possible to begin a sentence with 'been'; like in the sentence " Been a subscriber since 1994" ? But I know that using 'I have been' is what Iam more used to and grammatically accurate. So is there any actual difference between the two ? Also I believe 'have' was gradually omitted/swallowed over the course of time. Cause, I have been seeing people writing essays on youtube comment section beginning with this type of sentence...!!
In conversational English, and especially in social media, we often use ellipsis - "the omission of one or more words that are obviously understood but that must be supplied to make a construction grammatically complete" [Merriam-Webster].
So yes, I have been a subscriber since 1994 is grammatically complete, but in conversation it's common to omit the subject and auxiliary if the sentence is readily understood without them:
"Been a subscriber since 1994; been an active user since 1996; received an award in 2008; never regretted signing up."
However, be aware that ellipsis is a common cause of ambiguity or confusion, since what you might think is "obviously understood" may not be what your intended audience thinks. For example, with the subject omitted and with limited contextual clues, it can be unclear whether the speaker is referring to themselves or to someone else.
Regarding the second part of your question – omitting just the "have" but retaining the subject "I" – hence I been a subscriber since 1994...
In some dialects, and sometimes in informal English, people do indeed do this, but it's a bit riskier unless you know your audience, since it could be understood variously as: - normal vernacular in the local dialect, or - rapid or informal spoken English where a very faint pause might be heard after "I", indicating the "have" is understood but has been "swallowed", or - poor English, where the audience might assume the speaker has limited education or has limited fluency in English.
In formal writing you would never omit "have" here, as that would be both incorrect grammatically and not an acceptable ellipsis.