Yeah I've seen Nix and Guix suggested but they seem like a huge extra layer of complexity.

Also, strict backwards compatibility in APIs is totally worth it. It makes developing larger systems so much easier.

Usually not for first party code. It adds extra maintenance burden for little benefit.

For example suppose you want to add an extra parameter to a function. In a monorepos you just do that and fix all the compilation errors. Send one PR through CI. Job done.

With submodules... You have to add a new function so it's backwards compatible. Deal with a default value for the old call, maybe add a deprecation warning. Oh and you need to version your library now. Then good luck finding all the places that function is called and updating them...