I'm not sure teams really think that deep about this stuff any more except for a few all time great players. I know there was a point that the Saints equipment guy, Silky Powell refused to give out #8, and that tradition continues now, but I think the only number they really won't give out is #9.
I mean, if the Saints can give #57 to anyone then I'm not really sure it should matter who wears a specific number. I know there was stuff about the Saints quickly giving out #13, but to me they are just numbers unless the guy is a HoF player or a future HoF player, I'm not sure teams need to not give out the number. Even then some teams have so many HoF players that they have to give those numbers out.
I guess I get a bit of a "mourning period" but I'm not sure what length of time is appropriate.
Anyway, it's possible the Raiders did this because Carr essentially refused to be traded last year so they ended up getting nothing for him and are butt hurt about it. (Though they have no right to be since they negotiated the terms of the contract that forced them to cut him.) But, it seems more likely that it was just a matter of the number was available and QB number are at a premium now that WRs and DBs are wearing numbers from 0-19. I also find it odd that O'Connell wore the number last year and won't wear it a year after Carr left. Just seems like the "mourning period" for Carr should be over.