As someone who has been here for coming up on two decades (not a typo!), this is a pretty normal cycle post-AC game releases. And inevitably there is some discussion about TBT "dying" but it seems to be largely immune from the general trend of forum decline across the internet. My understanding is that TBT is in the top-five most active Nintendo-based forums; at least, that's what I've been saying when folks ask me about TBT. I'm not including juggernauts like Resetera, which splintered off from NeoGAF after allegations that their founder was a serial sexual abuser, and Famiboards, which itself splintered off from Resetera after some falling out among the Nintendo community there.
I don't think TBT is going away anytime soon, especially since AC is very likely going to be a tentpole franchise for Nintendo (it already is, really). I'm sure that the COVID lockdown gave New Horizons a sales boost, but the reality is that unless Nintendo colossally misfires with the Switch successor, it will continue the pattern of extremely strong software sales. And AC will 100% be part of that, as will then the inevitable uptick in forum activity here -- I've seen it happen with every single game in the series starting with the hype cycle leading up to the release of Wild World (of course, AC GCN had already come out by the time TBT emerged).
I find TBT's survival particularly interesting amidst the emergence of Twitter, Discord, Reddit, etc. I guess it's a testament to generational differences, but I never saw those sites as forum replacements, per se. They're great resources and interesting places to talk to others, but none of them engender discussion in the same way that a forum does, IMO. Clearly most of the world doesn't agree, so it's nice to see TBT trucking along. It's also a fun thing to tell people as I've gone through medical training -- "Hey, you know that Animal Crossing game that everyone was playing during the pandemic? I co-founded a forum focused on it that is STILL one of the most active Nintendo message boards on the internet." I'm damn-near as proud of that as I am of my medical diploma! Haha.