TMP used numbers for the decks fairly consistently. From TWOK onwards though, we have a large "A" on the Bridge doors, Spock's line about the elevators "they're inoperative below C-Deck" and two instances on Kirk's little turbolift ride (a "B" when he enters and a "D" or "O" when he leaves). Kirk also tries the elevator after being rescued and there is a "D" or "O" on the doors then.
I like the notion of a stylised "8" though, reminds me of Kirk's request for "turboshaft 8" back in TMP. Other rationalisations are possible too, but the real sticking point is the big "A"s on the Bridge. What on earth
could that mean?