I wasn't aware that anyone still used Yahoo or Google Groups. I've been subscribed to several, once active groups, but haven't received anything from them in years.
I guess, like anything that’s been running for so long, there will be a very high proportion of dormant things in it. But there are obviously a sizeable number still going, some very large. The small
Groups.io, which offers a paid-for solution for migrating a Yahoo/Google group and its archives (of messages, files, photos, etc) has had to double its prices to handle the work- and server-load of huge group transfers: "We are having these massive 5k-10k member groups transfer over right now (or groups with 400k messages)...” <
https://beta.groups.io/g/main/topic/36369511#22528>
So while some groups withered over time others, like survival of the fittest, carried on… and the bigger you are the harder it becomes to move to anything else, until that’s the only choice.
Phil