Which is exactly how CVS (and its predecessors RCS and SCCS) worked.

They were file based revision control, not repository based.

SVN added folders like trunk/branches/tags that overlaid the file based versioning by basically creating copies of the files under each folder.

Which is why branch creation/merging was such a complicated process, because if any of the files didn't merge, you had a half merged branch source and a half merged branch destination that you had to roll back.

The file lock was a fun feature when a developer forgot to unlock it and went on holidays. Don't forget the black hole feature that made files randomly disappear for no reason. It may have been the worst piece of software I have ever used.

The lock approach is still used in IC design for some of the Cadence/Synopsis data files which are unmergable binaries. Not precisely sure of the details but I've heard it from other parts of the org.

I remember a big commercial SCM at the time that had this as an option, when you wanted to make sure you wouldn’t need to merge. Can’t remember what it was called, you could “sync to file system” a bit like dropbox and it required teams of full time admins to build releases and cut branches and stuff . Think it was bought by IBM?