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That is a big if that can be easily broken by a new guy who don't know the rules or if the pace is fast enough that you cannot review everything.

If the codebase is different you can force the separation not just ask nicely to keep the code well modularized.

You can enforce modularity with git acls, and multi-module workspaces within a single git repository.

for example: golang - multi-module workspaces javascript - yarn workspaces

With this setup, you can create modules a, b and c. And then add restrictions about which which modules are available to a given module