See #432 (closed)
So, I expect this to be the last big merge request regarding tag support in the API. There will probably be a smaller MR specific to radios, an admin specific MR, and some minor tweaks here and there, but I consider the backend implementation mostly complete with this branch, from an end user perspective:
Extend tagging support to albums and artists
API to suggest tags on tracks, albums and artists (using our existing suggestion system)
Expose tags in artist, album and track representations
Enforce a (configurable) maximum number of tags per entity, to avoid performance issues or attacks
Time for user interface!