I agree with you here, though I think rather than removing the limit entirely we can just lean on the max_tags_per_topic setting? I think that would be safer.
Unfortunately, adding [] around the tags won’t help much with Gmail searching, only visually, since from what I can see (e.g. look at Create Gmail filter that contains a special character - Web Applications Stack Exchange, the documentation linked to is no longer there but it is still valid) Gmail strips special characters when searching in the subject and other places. Try searching subject:("[support]") in Gmail, you will get everything with support in the subject not just the ones with square brackets. This is kind of nonsensical of Google to do, they are a search company after all (well, search and ads) but there isn’t anything we can do about that.
We should also be able to easily add X-Discourse-Tags and X-Discourse-Category at the same time in MessageBuilder since we already have those options at this time, and as you say @mattdm this covers the web-resistant users nicely:
I can take this if you’d like?