Generally new radios will work fine with older servos and speed controls. One possible issue to be aware of, some new radios have high speed modes that require high speed servos (generally digital). Most can run in a lower speed mode that allows the old stuff to work.

I can’t remember but I think one of the high-end radios does require high speed servos.

I’m not aware of any speed control-ESC issues.

And define old, if really old, say back to the ‘80s, the servo plug wiring order may need to be changed, depending on the brands of the servos and previous radio.