You can't block font enumeration without crippling the entire CSSOM.
But that doesn't affect iOS, because you can't install fonts on iOS.
But that doesn't affect iOS, because you can't install fonts on iOS.
Custom fonts can be installed via custom configuration profiles[0], which is what some font applications do[1]
I'm not sure if this is exposed via Safari or not, so it could still be a moot point.
[0] - https://developer.apple.com/library/content/featuredarticles...