> Different projects depended on different versions of the same internal libraries, we had to...

This is one of the main motivations for using a monorepo with all third party dependencies imported all the way in, and only one version for everything.

Of course it causes a whole bunch of other problems and is kinda expensive to scale, so most places won't do it.

Isn’t this exactly why the Go team recommended vendoring deps for years and years before go modules came up? Even now it takes one simple command to vendor them.