I think for now the only proper documentary on imageboard users would have to come from themselves. There's an active effort to make imageboards look like nests of potential school shooters while some agitators want to embrace this.

Add that with many boards actively hiding themselves, most users being deliberately unreachable and obtuse, and archives being split among countless sites that have a date limit. This leaves you with a subject that's less "ask some redditors" and more "research some derranged nomads that never learned to write".

I watched a documentary on hobo culture once, and it presented a gentrifying group that gradually lost its usual stomping grounds. Something like that could happen decades into the future.