You can compile any old code with newer rustc as long as old project does not use unstable features that have changed/went away. What are you on? I recently recompiled project from 2015 with latest stable rust just fine.

All breaking changesin rust done via editions and you can mix-and-match editions.

I suspect GP is using system rustc to build random projects off of github and doesn't use one of the distros that actually updates rustc, like Fedora, SuSE or Arch.

Then the conversation is about forward compatibility, whether developers should wait X amount of time before using new std APIs or features, and whether the ease of using rustup and project expectation of it being accessible is reasonable or not.