It is not illegal for wheels to protrude past the arch.
If it protrudes more than 30mm, then it is illegal.
But at the same time all tyre must be under the arch.
Why people stretch tyres, to get a bit of poke, but still be legal (but then some people go mental and stretch to the point of stupidity and ruin it lol)
I have been pulled, spoken to a traffic cop, he then looked up what I said, and then left me to be, as I was legal...wasn't dangerous, nor catching the arch, so nothing they could really do.
Luckily the stretch was tiny (wheel was 8.5J which is 215mm anyway)