I really like monorepos. But I find that it's almost never a good idea to hide parts of a source code from developers. And if there's some secret sauce that's so sensitive that only single-digit number of developers in the whole company can access it, then it's probably okay to have a separate repository just for it.
Working in environments where different people have partial access to different parts of the code never felt productive to me -- often, the time to figure out who can take on a task and how to grant all the access might take longer than the task itself.
Working in environments where different people have partial access to different parts of the code never felt productive to me -- often, the time to figure out who can take on a task and how to grant all the access might take longer than the task itself.