Quite a minor thing but don’t you think that Clear Unused Tags in the Notes menu of the Browser could be renamed to something along the lines of “Refresh Tags” or “Refresh Tag List”?
Clicking it doesn’t only clear unused tags, but also, when you create a new tag, adds the newly created tags that were not present before. Does that make sense?
Let me know what you think 
P.S. The AnkiMobile app on the iPad is dangerous, thank you!
Now that you mention it, I was recently annoyed by new tags not being visible in the sidebar. You gave me the push to check this out and indeed: On my add-on version (2.1.22), new tags are shown immediately without further action.
I believe this is the intended behaviour, hence the naming, and there’s a bug in later releases.
Reopening the browser should update the tags list, but I think new tags also appear after a call to maybeRefreshSidebar. Anki doesn’t call this function after editing the tags field (I don’t know if this is on purpose), but call it for the Clear Unused Tags item and in some other situations.
I believe the item’s main purpose is to clear unused tags. Showing newly added tags is just a side-effect of refreshing the sidebar. Reopening the browser will do it as well.
I’ve added https://github.com/ankitects/help-wanted/issues/25
@andrewsanchez you mentioned you were interested in Rust tasks; this one is (I hope) fairly low-hanging fruit