Can anything cancel one of Ten Numb's crits? If not, couldn't the card text be shortened to "When attacking, one of your critical results cannot be canceled"?

It could be, yes.

It could according to what is possible right now in the game. But if a reverse Crack Shot ever exist, it would be able to cancel Ten Numb crit

Currently, yes it could be. Right now there is nothing that can cancel a crit specifically besides regular evasion, everything is hit specified (Draw Their Fire does not cancel anything and its the only crit-specific effect in this manner)

But like muribundi said the way its worded enables it to be cancelled through any future means. Some discardable "stealth" tech could cancel a crit perhaps.

I'm making an Alt Art card, so less words are better. There's plenty cards that don't reflect the faqs in this game, so I feel fine shortening the text for now. Thanks!

With Ten Numb you have to also remember that, due to a Rule Reference and FAQ inconsistency, when he was introduced to the game spending an evade token cancelled an attack die (per FAQ, Rule Reference said it "added an evade result") so it could be used to cancel his Critical Hit result and it was only a year or so later that FFG corrected the inconsistency and universally made evade tokens "add an evade result" to the defense roll.

In this instance we have an upgrade card with over clarification and, given the number of cards that lack clarification from the newer waves, we should be thankful for it.

FAQ page 8, Timing Chart, 7.ii “Remove shield tokens to cancel [crit] results.”

Omiting the defense dice stipulation on Ten Num’s pilot ability, someone could make the argument his crits bypass shields.

If it did not include the "defense dice" stipulation, you could run Ten Numb with a Mangler Cannon and an Accuracy Corrector , convert one hit into a crit, then cancel dice all except the now-uncancellable crit, and add 2 hit results to your roll.