Welcome back!
There are definitely highs and low in terms of forum activity. New Horizons, launching in the early days of the pandemic when there was a lot of uncertainty, confusion and lockdowns, had a big boom period since everyone and their grandma bought it. It was very popular for a while, but as someone who was on the forums during the New Leaf era, it seems like the hype around New Horizons was considerably more short-lived. With the piecemeal content updates dripfeeding content to the game, a game which had less engaging things to do than New Leaf and not as much to do with friends online, it seems like forum activity had already dipped after the first year and I don't think even the 2.0 update and the Happy Home Paradise DLC affected much of anything from my vantage point. I don't know the numbers, but I didn't necessarily notice another wave of newcomers or anything, or at least I don't remember it. New Leaf, comparatively, seemed like it was a longer bleed of interest dropping off, whereas New Horizons seemed to pour out basically all at once. This is just speaking to this forum, though, as I only barely used Nookazon in the early days and don't know what sort of sustained levels of popularity there might be among the fandom at large.
Forum activity always bounces back with the release of new mainline games, but we haven't had one since New Horizons in 2020 and don't know when the next one will be announced, much less released. So yeah, compared to those periods, we're in a lull.
That said, there's still a solid community here. Some people are still interested in the game itself, whereas others aren't so much after all this time and prefer to hang out and discuss stuff in the other sections like Brewster's Café, Nintendo Treehouse and Gamers Lounge' or play forum games in The Basement. Some make friends and/or form friend groups. Some get into the forum collectible scene.
There's certainly still activity in the Animal Crossing sections, but whether it's sufficiently to your liking is another question. However, these forums still have plenty to offer in other areas. The forums have various events throughout the year to keep the community engaged, having fun and giving us things to look forward to. You can see what sort of forum events are on the horizon throughout the rest of this year
here, in the February 27th Bell Tree Direct thread. There's currently a
flower collectible breeding event that will unfortunately be ending tomorrow, but as you'll notice, there's plenty still to come throughout the rest of the year. The TBT World Championships, the next event, will be competitive gaming in things like Super Smash Bros., Mario Kart, Splatoon 3, etc. The Bell Tree Fair, starting in late July and running through to late August, is always a big, popular event with plenty to do. You ought to check out some of the events when they roll around. They're always posted in
The Bulletin Board section up at the top.