Resolution: Works As Intended
https://api.mojang.com/users/profiles/minecraft/u returns 204 No Content, but according to https://sessionserver.mojang.com/session/minecraft/profile/c6893ba47cfd4002abea33d4b1bd3f7a that user still exists.
The same applies to user Z (ca58c68cf220499b944926cb4f25c2c6), and perhaps more users, see comments.
I am aware that the current limitation for NEW usernames is 3-16 characters, but old accounts don't have this limit. See for example https://api.mojang.com/users/profiles/minecraft/G or https://api.mojang.com/users/profiles/minecraft/ez.
This is likely related to