If Salladhor Sahn had never shown up again following his second appearance, his part in proceedings could've easily been considered finished. The acquaintance of Davos Seaworth first appeared in season two, where he pledged his ships to Stannis' cause in exchange for gold after an awkward conversation (invented for the show) about raping Cersei.
He was not seen during the Battle of the Blackwater, but presumably participated in it off-screen, given that he popped up once more at the start of season three to save his old friend from the depths and deliver him back to Dragonstone, leaving shortly after in the belief that his contract had been fulfilled despite Stannis' loss.
The fourth season saw him once again show up in a singular episode, however, as Davos sought out his services for Stannis' cause once more after they were successful in negotiating a loan from Tycho Nestoris of the Iron Bank of Braavos. With Stannis' stubborn personality, it's certainly surprising that he would've entertained engaging somebody who'd absconded from him before, but the hiring was never mentioned again.
It could be easily interpreted that Sahn abandoned Stannis once again, but it seems questionable to have introduced him twice without having him do anything.
Nestoris reappeared in season seven, where his poor investment wasn't addressed as he opted to pledge his Bank's coffers to Cersei Lannister instead. How he's still in a position of power after that is anyone's guess.
What do you think happened to these characters? Will we ever see them again? Share your thoughts down in the comments.