"The Man" is slang for the government, or an authority, commonly with negative connotations.
Your sentence effectively means:
Ideas/changes were implemented in response to what people saw as the authority/power imbalances at the time. However, now those are the new "normal" - those ideas (that were previously considered radical) are being seen as the authority/power systems that need to be challenged.
a slang phrase that may refer to the government or to some other authority in a position of power
- the Man , somewhat old-fashioned : the police
- the white people who are seen as having power in the U.S. : the white establishment
the person having power or authority over one; esp., as orig. used by U.S. blacks, a white man
It refers to the people, or systems of power, that are seen to control you (or society). It is generally used with negative connotations - that they are the authority which holds you back, or in some way is causing problems in your life.
the Man is keeping me down
Which refers to how the people in power, or systems of authority - are preventing somebody from succeeding in life.
It's relatively hard to track back a good origin of the phrase, as it has been used by numerous groups through the years. Including (but not limited to):
The early 20th centuary underworld crime ring - where "The Man" derived from "The Boss", who was the prison warden, and then over time - anybody/system association with him and the police.
African-American culture, especially during the civil rights era - referring to "the white man", who was seen to control society (as at the time, the government and authority systems were almost entirely comprised of white men).
Hippie culture, from the 1960s, where "The Man" referred to the govenment and authority systems that were seen to be reducing people's freedoms.
Note that these uses of The Man are not the same as the approving/praising phrase "You're the man!".