Source-available is just as bad as proprietary as it distracts from the freedom that open source/free software gives. It also undermines open source by confusion which you are trying to clear up right now. Don’t legitimize source-availability
That’s only true if you’re talking about the goals of open source/free software generally.
If we’re just talking about a game engine and releasing games, being able to modify the engine is absolutely critical when optimizing a large game. So having source available is absolutely a very practical thing when using proprietary software.
So it really depends on what you’re concerned about. Source available is just as good as open source in most cases if your goal is to build closed source software. If your goal is to build open source/free software, it’s awful.
In most cases you are NOT allowed to modify source-available code, just to look at it
I’m pretty sure you can always modify code for personal use, you just can’t always distribute those changes. In the case of a game engine, this would mean you could modify the engine code in development, but you could not release your game with those changes in.
Unreal allows modification and distribution, but only if you’re a licensed user and only for your combined work, but you cannot distribute your own fork of Unreal, aside from a patch set for other developers.
