I ran a couple of batch jobs to close out way old support topics created before we had auto close, and also archived (= freeze against future modification) some topics from 2014. The latter had the unfortunate side effect of marking those topics "unread" again, including things like welcome messages from the forum itself...
I won't be doing that again, obviously.
In the meantime, the
Unread -> Dismiss... path may be useful for those of you who were all read up before now... Sorry about that.