Thank you for the update and the effort on fixing up the old packages!
-Wimplicit-function-declaration, -Wimplicit-int, -Wincompatible-function-pointer-types all default to an error in Clang 16. -Wint-conversion was defaulted to an error in Clang 15.
Not warning on main() is something we could certainly consider, but the only situation in which we warn on int main() is when the user explicitly enables strict prototypes and that definition of main() has no prototype, so it is also reasonable that we fire the diagnostic. -Wstrict-prototypes is off by default. We did add a new warning along the lines of what you’re asking for (-Wdeprecated-non-prototype), which is on by default: Compiler Explorer and modified our behavior of -Wstrict-prototypes to warn pedantically about use of functions without prototypes (we did this during Clang 15). Does this change your opinion on where things stand?