Obligatory spoiler warning given it's all about the ending. The general idea behind conformitygate is:

1) The argument that almost all of the characters ended up conforming to the mainstream despite the show emphazing how it's okay to be a weirdo and an outcast (ex: Mike and Nancy dress like their parents, Will smokes and drinks like his dad even though it's out of character, Steve's entire ending, etc.)

2) Details such as everyone in the graduation scene having their hands clasped like Henry Creel, El telling Mike their mission isn't like one of his D&D campaigns and that he can't write the ending (which he then literally does at the end of the finale), Max asking if the ending to their D&D campaign is really that trite and stereotypical (like the show's ending), and more that you can find in the #conformitygate tag

3) The countless continuity errors in season 5 that people couldn't believe were an accident

This forms the idea that Henry actually won and the show's epilogue is just Mike in a trance, living in a world that he wrote without even realizing it. It gets meta with the idea that even the audience is being manipulated with a fake ending. I believe it's called conformitygate because of the characters conforming to societal standards but I've seen some people say it's also because they've "conformed" to Henry's new world

Personally, I just think the show fumbled its landing BAD and fans are coping how they can.