Where are these other communities? I can’t find them. And no, I don’t count Discord as an acceptable replacement. Googling for old-school forums has been pretty fruitless for me thus far as Google is basically useless without appending site:reddit.com these days (the supreme irony of that…)
I guess it depends on how niche your interests are. I’m mainly into electronic music machines and synth diy. There’s enough forums for me. There’s other stuff that I frequent on Reddit but at the end of the day it’s usually just crap to distract me that I could do without.
I'm part of a lot of smaller women-centered communities on reddit - my favorite subreddits were r/MoneyDiariesACTIVE, r/FIREyFemmes, r/girlsgonewired r/adhdwomen, r/AmateurRoomPorn, r/AskWomenOver30, etc. Not all of these are explicitly for women only but they all have a majority female user base. I don't think there are equivalents in a forum-type format; I'd probably have to go to TikTok or something, but I don't care for that format so I'll probably just lose those communities for the most part.
The big "mostly women" subreddit I'll miss, despite being a guy, is the /r/RomanceBooks subreddit. It is hands down the best book community on reddit. At least as far as community and moderation goes. It sounds like the mods don't think the alternatives are technologically mature enough.