Looking at all the answers in this thread, i realised I might miss Reddit (subreddits) more than I thought.

noncredibledefense I didn't understand all of the memes, but I loved the energy.

I used to love lurking on AskReddit because there was a lot of interesting stories and topics there

datanalysis, userexperience, uxdesign, digitalmarketing, extramile, watchitfortheplot, coffee and combatfootage

Linux_Gaming, PCMR, EscapeFromTarkov, NobaraProject, My city's sub.

Leafs. It's either that or Twitter, which I will certainly not use.

Luckily there's a Sports bot on Mastodon that duplicates Twitter posts from NHL and Leafs accounts. Will be good enough for me.

COLLAPSE

Call me a doomer if you want, but i think we're gonna be way fucked a lot sooner than expected climate-wise

Sunrise and Sourdough

There needs to be a comedy podcast aggregate community. Cumtown/The Adam Friedland Show, Matt and Shane's Secret Podcast, YMH, Bad Friends, Rogan, MMP, TFATK (homeless cats only), etc, etc...

/r/aoe2 was one of my favorites. I hope plenty of users decide to make the switch.

r/tooktoomuch

I am going to miss my programming subreddits and r/halo. A lot of good folks over there.

A bunch.

r/guineapigs - it was very helpful for new pig owners, and a great guinea pig fix when you're at work.

r/disney* - for a Disney parks nutcase like myself, the various Disney park related subreddits helped me get over the time between visits.

r/bestofredditorupdates - I get frustrated reading posts and not having any idea of how things ended up, but boru allowed me to indulge my interest in others' stories without getting frustrated by the lack of closure.

The list goes on and on.

Firefox. AndroidGaming, Android, Windows11,Microsoft, Chrome, SmartLauncher.... lots of various speific support communities. Also valuable ones like AskDocs, AskElectricians, etc. Reddit is a really good way to find advice from people that know what they're doing (of course you have to be careful).

Biggest ones will probably be r/bouldering and r/climbharder.

The tech related ones will be more likely to have big replacements, but bouldering is pretty niche, so I'm not sure about that one.

SummonerSchool and CompetitiveTFT, I know there are discords but I don't like its format.

nba, sffpc, pauper, magictcg and some other niche things.

artisanvideos. I love a good japanese watch maker making a wooden cabinet in a rainforest with rocks as tools.

Professors, Watercolor, Technology, Privacy, Brogress, all the Star Trek subs. Ghosts and Paranormal. Nudism and Naturism. Fountainpens.