They fizzled out, members probably moved on to various other groups and projects, while the rest simply went on with their lives. A danger of being decentralized is losing all of your momentum.
Not gonna dig through their Twitter feed, but I saw someone a couple months ago ask them this exact question on one of their posts, and they wrote a pretty interesting response. They basically said, we're still here, trying to fuck the system up, but, with all the information we've provided and ported out there to the world, y'all haven't done dick with it. Laws haven't been passed, politicians haven't been ousted, corporations are still abusing the systems. So they were basically saying, what good is them leaking and hacking if the public doesn't take a more activist approach towards change themselves and hold the people they expose accountable.
Well if I knew how to take down sites and child porn site I totally would. Just don't know what to study and probably don't want to be another computer cracker using programs found online.
Hacking got harder, and the enforced penalties for getting caught became a lot more severe (in the west at least). This meant that most hackers aren't doing it for luls but for serious business.
They (I use that term to mean the average 4channer) were co-opted by alt right propaganda.
Most neckbeard, incel, Andrew Tate followers are what Anon originally was. We just lied to ourselves that it wasn't really racist and that we were fighting a good fight.
Now, its a bunch of sad lonely people that found acceptance in intolerance and hatred.
it's also important to keep in mind that the cybersecurity field has adbanced tremendously, with cloidfare, EDRs, and in general it is now way harder to do anything anonymously without getting caught, quickly. This also males the field of hacking way more difficult to get in, which combined with reduced attention span of younger generations probably means there's not that many bored teens willing to put the time in, and as an adult you have way much more to loose, so for hose who had the skills it would be a lot greater risk.
Curious to know what answers you were looking for here OP. What makes you think they aren't doing these things but stopped identifying themselves as such? Also some dialogue is required about the myriad of things 'Anonymous' took responsibility for but were never adequately confirmed as objectively true--and more importantly, what is accomplished with this last feat? I believe the answer you seek lies in these depths