[dtech]

Only v6 and v7, v8 is "whatever you want".

[mcv]

That's not much of a standard.

[throwawaymobule]

From my reading, it's standardized because it has a version number embedded, so it won't get confused with other uuids.

Probably a good idea to have somewhere to silo bespoke implementations. Just gotta hope they end up being unique.

[dtech]

It's not uncommon to provide a way how to do it in standards because it will happen whether you want it or not. See e.g. HTTP headers starting with X- or MIME types in application/vnd.