I've damaged previous Teensys that weren't 5v tolerant (Teensy 3.0, Teensy 3.6) with accidental 5v inputs. Usually the pin becomes unusable, but the rest of the Teensy limps along.
Teensy LC, 3.2, 3.5, 3.6 have special ESD protection circuits that do not look like a diode between the pin and 3.3V. This is sort of a mix blessing. It lets you more easily drive the pin higher than ~3.9V, but if you do, there isn't a conductive path to the 3.3V power line, so just 1 pin takes to take the abuse.
Teensy 4.0, and also Teensy 3.0, do have diodes as their ESD protection. Perhaps that protection is more robust? Difficult to say, since Freescale (now NXP) doesn't disclose much info about the ESD protection circuits. It's probably a closely guarded trade secret.
The main consequence of a diode-based ESD protection is driving the pin very hard (low impedance) can put a lot of current through the diode. Most modern chips don't have
latch up problems, but that could happen. You could also pour in enough current to possibly power the entire chip, raising the 3.3V power up higher.
The other side effect of the diodes is the possibility of a phantom power effect if you drive the pin while the power is turned off. Defragster saw quite a lot of this during the beta test. It's not an issue we can solve in software. It's a matter that the pins do have diodes.
So the behavior of Teensy 4.0 when abused will be somewhat different than most of us are used to with Teensy 3.1 to 3.6 and LC. We haven't had a 3.3V Teensy with diode-based ESD protection since the original Teensy 3.0.