I saw 3 usernames on NameMC that have periods in their names. They are all unmigrated but people still use them becuase the first time i looked at one of them they had an earth skin but now they have a regular person skin.
Usernames:
https://namemc.com/name/.Flow
https://namemc.com/name/Mr.LoL
https://namemc.com/name/internet.com

The uuid is in the mojang system.
https://api.mojang.com/users/profiles/minecraft/.Flow
https://api.mojang.com/users/profiles/minecraft/Mr.LoL
https://api.mojang.com/users/profiles/minecraft/internet.com

I think the people who made the names made them when name restrictions were not done yet.