I’ve been following this topic and am also interested in it for our own purposes here on meta.
It is indicated as solved by Applying SEO settings to tags - #20 by pento but that only makes the tag description show up in the meta description for the tag page. Also it appears to not be working reliably. Damian started Topic Tags SEO meta description stopped working to ask for that to be looked at.
As I understand it, the ask is to make it possible to specify four distinct things for tags:
- tag title
- tag description
- tag meta title
- tag meta description
(2) already exists, and if specified shows up on tag pages, and is also served up to search engines via the meta description on tag pages (4).
To optimize tag pages so they work well interactively on the site as well as showing up attractively in search results so people actually click on them, you want to be able to edit all four of these individually.
Category pages share the same problem.