There are still proper open core ones out there. When it comes to closed source* I prefer source unavailable except for security auditing reasons, because I don't want to be accused of stealing their code. I am consistent about not wanting to look at their closed source code, so if get accused of that I will vigorously defend myself from it.
I could use a script to git clone without a directory so I can view these offline. :)
Some have empty ee directories set up in anticipation of doing this in the future.
I don't know if it's to trick people into becoming tainted by it or to increase acceptance, but it no doubt tricks people into downloading and viewing source-available proprietary code. It's one feature of a monorepo that I see as a disadvantage that has further had me be more of a microrepo guy. :)
If it's source available for a security reason I prefer it to be in its own repo like Tarsnap. https://www.tarsnap.com/open-source.html Here's the source available one: https://github.com/Tarsnap/tarsnap Here's the list of repos with some various open source ones to show it's open core: https://github.com/Tarsnap That's a proper Open Core repo.
GitLab FWIW follows, maybe pioneered, this ee directory model, so though there's a lot I like about GitLab they get this thing about Open Core wrong IMO.
* You can see the source, but it's still closed in terms of what you can do with it, so I am comfortable using "closed source" and "proprietary" interchangeably when talking about source available code.