@waha06x36 @Gargron <a href="https://some.example/tags/microformats" rel="tag">verbose microformats category tag</a>

»Tags are embedded in HTTP URIs in a well-defined manner so that the tag embedded in an HTTP URI can be mechanically extracted from that URI. Specifically, the last segment of the path portion of the URI (after the final "/" character) contains the tag value.» - http://microformats.org/wiki/rel-tag

There you go. !ostatus doesn't require it, but HTML generated by !gnusocial aims to honour #microformats2 since it's great and makes us part of the !indieweb community.