One company I worked for did the enterprise version through a git submodule. So main repo was the OSS version (with it's own main), and the git submodule was a different program entrypoint that contained / activated the enterprise code. So devs would have both the OSS/Enterprise module checked out and could work on both together.

In this example as I recall most of the enterprise code was just activation code, so a properly motivated individual could probably figure out how to turn on some of the enterprise features, but most people won't bother to figure out how to do their own builds, and it's more work then finding the secret env variables in an open repo.

It's a little bit painful at times because changes that cover both code bases need two commits, and it isn't uncommon for someone to forget or commit the wrong submodule pointer. But depending on the team, it can work.

Not for everyone, just including it as an option I haven't seen mentioned in other comments.