I loved SourceSafe for the disappearing files, and the usual "John locked the file and he went on holidays."

Well that was just bad manners and you could force unlock.

Perforce also did exclusive file locking (and I believe Microsoft ended up using a customized Perforce internally at some point).

sd was a light customization of p4

https://devblogs.microsoft.com/oldnewthing/20180122-00/?p=97...