F.4 should only apply to libraries

F.4: If a function may have to be evaluated at compile time, declare it constexpr

Please don't. The compiler will tell you if and when you actually need to declare it constexpr. Instead of pondering and guessing about this ahead of time, and writing longer code for no reason, just add the keyword when needed.

The one exception to is libraries (of any form). Library interfaces need to be thought out ahead of time, because you don't want library users changing the code.

Start by locating guideline F.4 in the C++ Core Guidelines content and read its surrounding explanation. Update the rule so its applicability to libraries is explicit, while preserving the stated rationale; done means the guideline clearly distinguishes library interfaces from ordinary application code.