Apparently, should Swansea and Hull finish joint 18th, level on points, goal difference and goals scored they will play off at a neutral venue to decide who goes down. https://www.theguardian.com/...-relegation-play-off
There's a few games to be played before we arrive at that scenario. Middlesbrough might have a say for starters.
And why don't they use head to head records as the next tiebreaker before having to resort to playoffs?