Anyone can add tags, TL1 users can create new ones.
Yes, but can they add them to posts that they cannot edit? (Say, because the posts are too old or written by someone else?)
This is something we already discussed in the context of adding tags to content migrated from the old forum, but so far, I’ve seen no reason to assume that tags are treated differently from other edits to a post.
My understanding is that yes, it should be possible.
(veering offtopic somewhat, but)
are you sure that that’s correct after the initial post? I’d do that via the “edit” pencil on the post title, but we’ve had users elsewhere saying that they’re not able to do that. I’m guessing that’s a low-trust thing, and an anti-spam measure that’s there for a good reason.
Sorry I messed up moving the messages to a new topic and forgot to move the initial message, and got it at the end of the new topic.
Just as a quick test, as a “Trust Level regular”** user I was above to add a tag to https://community.openstreetmap.org/t/is-the-weeklyosm-alert-choking-low-volume-mailing-lists/1849 by editing the title.
** that was well-hidden in the profile! It took me ages to find!
I’ve tested this by creating a new user on the forum import test instance. I was only able to change tags of an imported topic written by someone else if my user had trust level 3 or higher. I have no easy way to test editing your own topics’ tags with a low-trust account.