i've done so multiple times, including 20 -> 19. going to depend on rng with ticketing, but some factors to consider:
1) have you ticketed for delevels before on that bnet account? (i'm assuming you haven't)
2) time /played at each level? the lower the time at the new level, the better, as it'll support the claim of it being accidental
3) how long ago did you accidentally level the character?
4) have you purchased any character services (faction change, server transfer) on the character in question? this too will increase likelihood, especially if you play the "faction changed, so my xp got turned on & i leveled by accident" card (not sure if this is even a thing still, but i did that a few yrs back for 20 -> 19 for eyepatch on a hunter. fairly certain i myself swapped xp on for bg farming, but the ticket succeeded)
either way, it's 100% possible -- you're just going to have to be persistent