One of my friends asked about this over a year ago in support, but I figured it’s been long enough that I might as well put in a feature request for it. I can’t imagine that it wouldn’t be useful to pretty much every Discourse forum out there one way or another.
Being able to group and store PMs in folders would be a really nice Quality of Life feature to prevent users from having to dig through tons of PMs to find the ones that they are looking for, which becomes quite the hassle if you are working on projects, playing forum games, or simply send a lot of PMs (or all of the above!). This is one of the most useful features of phpBB, and I think it’s well worth the time and effort to bring it to Discourse.
Technically this would be a very complicated thing to do, permissions get tricky. What if user Jane can not see any messages with the tag #this-is-between-bill-and-bob
Do we let Jane know the tag #this-is-between-bill-and-bob exists?
Do we let Jane know that there are 17 messages she can not see with the tag #this-is-between-bill-and-bob?
Clearly the answer to both is no, but to make it technically work it becomes massively expensive to get the list of tags and counts.