There is enough input impedance to not smoke out the clamping diodes.
As soon as you put the GPIO pad in output or ADC mode, with 5V being applied, they're smoked.
Note that oscillator and clock-out or Vbat domain pins are often excluded from being FT.
You didn't read my question, but if you need to apply raw +5V directly to a GPIO pin, use a current limiting R at least.
Anyho back to initial question, ADC (post clamp diodes) switch Rds seams to be 6kohm max according to data sheet,
this does not answer why some devices have GPIO pin mapped on ADC that are FT while other devices with the same ADC is not.
BOOT0 pin are +9V tolerant, a surprise.
But i wonder if it's just the regular ST sloppy documentation work again...?!