A private copy constructor can still be copied in the class itself. If you omit the definition, you still get a linker error message, but linker errors are among the most obtuse to actually discover. (Where did the copy construction actually happen? Who knows! You get, at best, the function that called it).

A deleted copy constructor produces better error messages. And it's not really backwards-incompatible at this point--deleted functions exist in compiler versions old enough to get a COVID-19 vaccine.