My first thought would be that they accidentally blocked Google's crawler (maybe through some kind of anti-AI setting?) or that Google believes that the site is serving malware or spam. Either scenario can have that kind of effect. I can see that their forum at least appears to have strong Cloudflare anti-bot rules in place, so that might be the case.
They're also using a subdomain for both their wiki and forum, which Google has been observed to punish. They might consider moving each of those to their own separate .com domain.
But aside from that usual stuff, there's one more possible reason that's specific to this site. In November of last year, the Pokemon Company rebranded their "Pokemon Trainer Club" to "Pokemon Trainer Central", which is the first result that comes up when you search for "Pokemon Central".
That change was made a few months before the sudden drop in traffic, but could still be a viable explanation here. Google does routine re-ranking on a daily basis along with occasional major re-ranking, which happens maybe a few times a year, so the delayed hit that they saw could have come from Google finally recognizing that most people who search for "Pokemon Central" are no longer looking for the wiki like was once true in the past.
https://gonintendo.com/contents/54863-pokemon-trainer-club-r...