Firstly, there's a very good chance driving without a speedometer is illegal in your area.
Secondly, anything that affects functional safety should itself be vetted fron a FuSa perspective. I guarantee you that most manufacturers treat the instrument cluster as a Functional Safety item. If CarPlay can't bring the system to a state where that analysis is affected (which seems unlikely here), there's no issue.
You shouldn't crash, but it's not unheard of for the speedometer to fail.
Certain early-2000s GM speedometers were prone to breaking. Either the servo motors straight-up stopped working, or the label started peeling up and jammed the needle. My brother had several trucks from that era with this problem.
Secondly, anything that affects functional safety should itself be vetted fron a FuSa perspective. I guarantee you that most manufacturers treat the instrument cluster as a Functional Safety item. If CarPlay can't bring the system to a state where that analysis is affected (which seems unlikely here), there's no issue.
That said, I'd probably use it.