Nothing is stopping you from passing different -std=c++XY arguments to different compilation units in your codebase. I don't know about other compilers but object files compiled with msvc, gcc and clang (the ones I happen to work with) using different -std values (starting with c++11) are compatible* and the respective teams working on these compilers reportedly make conscious efforts to make sure this remains the case.

CMake for instance makes this very easy with its set_properties() call.

* Of course there are caveats and corner cases. See eg. https://stackoverflow.com/questions/46746878/is-it-safe-to-l... But these seem like they could easily be avoided in real-world use-cases.