Official documentation

[Rudxain]

https://docs.github.com/en/enterprise-cloud@latest/admin/managing-iam/iam-configuration-reference/username-considerations-for-external-authentication#about-username-normalization

GH still has no dedicated doc page for user-names, but it's better than nothing.

BTW, it's worth mentioning that user-names share the same name-space as endpoints, and therefore any collision is invalid as it would create ambiguity. This is why notifications and join (and many others) are server-side invalid.

I understand that checking for collisions is out-of-scope for this pack, as collision-checks require API requests, which are non-deterministic and non-idempotent