Hugely unlikely (I'm a teacher) that a one off incident of assault - & spitting is assault - would result in permanent exclusion of an otherwise model student.
It's possible that it was seen as such an egregious & revolting act that it's an instant fixed term - not permanent -: exclusion, & frankly, that would be fair enough.
I teach a spitter. He has some serious SEN & additional behavioural needs & we have a risk assessment in place, because he does put people at risk when he gets frustrated & gobs at his peers or at teachers, which happens roughly every couple of weeks.
He's aware that it's unacceptable behaviour but he really, genuinely, struggles not to lose it, & when he does, he spits at people. Given the chance, he bites them.
& he quite often finds himself in the inclusion unit, which he understands. He knows that he can't be learning alongside the rest of his class if he's spitting at/biting people, & he's aware that he's messed up. He's improving & getting his head round it.
Tbh, OP, if there's a potential additional SEN reason why your dc spat at a classmate, then that's worth investigating. If there isn't but it was an unprecedented moment of rage, then that needs looking at too, because it's honestly a disproportionate response to being annoyed by someone. If there's a history of your dc behaving aggressively, then you should have been involved in discussion with school about the behaviour long before this.
I would be very much querying a permanent exclusion.