Lol’s boards closed march 9 (of 2020)
Smite closed their forum on July 14
Playdota (dota 2) fell apart in 2014, had a kick to try to get it going again as the ‘official forum’ again in 2016, and died again later. Not sure at what point there, but I find some inquiries in 2019 asking the fate of dota forums.
Strife was shutdown in october 2018
Vainglory lost official support in April of 2020
– though I think both may still have fan/community ways to log in, but what relics of their fan forums (not official) are ghosttowns with bot adverts making occasional topics from what I found.
Some of the tencent mobile moba games I haven’t followed, and they probably don’t have a “forum” at all, but I didn’t look beyond the first non-sponsored search hits I got cuz I don’t care enough to think they would actually have forums anyway (due to forum decline of other games)
Most other communities (such as Lol and Smite) cited low activity (a big drop-off of forum usage) in recent years as the cause of them shutting down their community resources. More players are involved in reddit or discord groups which aren’t really designed for extended conversations or archived information, but part of the decline of forums is due to people not caring about that sort of thing anyway.
Games offered on steam have the product discussion page (or even a dev page, but that’s for bug reporting even if it looks like an old forum) and some try to use that as a replacement with topics commonly made to complain about smurfing, rigged matchmaking, and what others may call ‘low elo’ complaints that commonly plague the genre communities.
I could suggest that the decline in people being willing to have a communication medium that exceeds twitter’s character length as being related to the decline in a more formal/letter-like medium, and I could even suggest there’s a correlation with that decline and the type of complaints that commonly plague the genre by groups identified by similar ranges of ability (or elo/mmr) but it’s not like I’d have some sort of official data to impose and just like to assume there is a connection between people having more patience (delayed gratification) compared to those that don’t.
Or I could just say that “memes” ruin everything 
I mean, i’m not really making a statement on that, I’m just poking fun of social media trends cuz I’m an old forum grump and can be all “well back in my day” as a gimmick of making this post even longer to try to get people to suffer that much more if they look at the length of it exceeding the demands they have for just wanting the tl;dr on “other games” forums. 