It wasn't mandatory, actually. It was possible to turn off the locking. But that was like entering a whole new world of pain. So most people didn't dare turn it off.

They called it "multiple checkout", and the manual strongly recommended you do not turn it on.

The software had issues with corruption due to operations not being atomic.

The lack of atomic commits in VSS, CVS and ClearCase caused absolutely horrific problems, even on small teams.

I remember when you basically had to yell "Committing!" and "Done committing!" to the whole room. Woe be unto those companies that tried to use VSS or CVS with more than a roomful of developers.

I can live with the locking behavior, but VSS had a tendency of corrupting the entire repo ever few weeks that is hard to accept.

I could live with the locking or the corrupted repos if it was half decent at just sharing code, but daily we had problems where someone would check in code, VSS would say it was checked in but the person updating would get stale versions. Sometimes if you looked at the server version it be be correct but not what you would receive on a fresh checkout.

It was literally less functional than copying to a shared drive. Of course management and other developers refused to believe that OSS solutions were better, MS ecosystem developers were an extremely insular group.

Yes, there was that too. I've seen quite a few regressions caused by this.