I think I read somewhere that game dev teams would also check in the actual compiler binary and things of that nature into version control.
Usually it's considered "bad practice" when you see, like, and entire sysroot of shared libs in a git repository.
I don't even have any feeling one way or another. Even today "vendoring" cpp libraries (typically as source) isn't exactly rare. I'm not even sure if this is always a "bad" thing in other languages. Everyone just seems to have decided that relying on a/the package manager and some sort of external store is the Right Way. In some sense, it's harder to make the case for that.
The better organised projects I've worked on have done this, and included all relevant SDKs too, so you can just install roughly the right version of Visual Studio and you're good to go. Doesn't matter if you're not on quite the right point revision or haven't got rough to doing the latest update (or had it forced upon you); the project will still build with the compiler and libraries you got from Perforce, same as for everybody else.
Sure, there are nuances to it as the need for ReproducibleBuilds [1] project demonstrated or people proving this concept of "bootstrapped reproducibility" to the extremes like Guix' full-source bootstrap [2], but I believe the fundamental understanding is the same.
[1] https://reproducible-builds.org/
[2] https://guix.gnu.org/en/blog/2023/the-full-source-bootstrap-...
No arguments here, it makes perfect sense to me as a practice. It's shortsighted to consider only "libraries" (as source, typically) to be "dependencies"--implicitly you're relying on a compatible compiler version/runtime/interpreter (and whatever those depend on) and etc
What was the nature of this project? Was this something related to game development?
That seems to be the only domain where this approach is used (from what I've heard).
I will admit that even though I think it might be worth doing (and not doing it because your version control system doesn't make it easy is the worst reason for not doing it!), I'm not sure I'd consider it absolutely mandatory, certainly not for video games anyway. Most projects I've worked on haven't done this, and it's been fine, especially if you have a CI system doing the builds - and now the individual developers' systems just have to be roughly right. But it is nice to have, if you've got the right combination of personpower and institutional will to actually do it. It eliminates a whole category of possible issues straight away.
(I'd like to think that for safety critical stuff like medical or automotive this sort of approach, but done even more carefully, would be par for the course...!)