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 https://webapps.stackexchange.com/questions/52828/create-gmail-filter-that-contains-a-special-character, 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?