It is a mistake to think that every pronouncement from a doctor is fully informed*. He’s simply wrong on this. That you hear the myth from multiple people is merely testament to the fact that it’s attractive for people who’d rather not wear steel toe boots. Just like anti-helmet wearers will confidently believe isolated anecdotes that a helmet makes you less safe, while ignoring the mountains of controlled studies with evidence that they make you more safe.
Steel/composite toe boots are required to meet certain testing standards (ANSI) for compression strength. They are required to withstand a weight of at least 2500lb without collapsing. If you have such a crushing injury >2500lb (let’s say, two horses jump on your foot at the same time) it is impossible to state (and implausible to believe) that your toes would have been salvageable if not for the pesky toe cap.
Note–Steel toe footwear does not protect your carpal bones. If that’s a risk for your activity/occupation, you can add spats (which are easily removed once that task is done)
*For example, lots of doctors who aren’t inclined to sports will very authoritatively tell a runner to “stop running” when something’s injured, or say “no running for 6 months”, when meanwhile a sports doctor will approach treatment from a perspective of “what do we need to do to get you back out there in 2 months?” They’re both doctors with the same training, just different biases and probably different commitments to staying current on latest medical info.