The tag search example in the documentation (https://musicbrainz.org/doc/MusicBrainz_API/Search#Tag) gives an example search query of

http://musicbrainz.org/ws/2/tag/?query=shoegaze

However this URL prompts the user to log in (in a browser I get an http auth prompt). Logging in with my MB credentials does work, but I'd expect this to not require auth, like other search endpoints.