I don't think anyone has returned to Twitter from Mastodon, of course most people didn't migrate in the first place, but Mastodon is much more active than it was a year ago.
Like the parent comment said, those names never left to begin with
> I run across lots of accounts that have 2 or 3 posts, then stopped.
yeah, you can do that with any random social media account. The average Redditor barely comments.