Wasn't it 2 automatically relegated and promoted with the 3rd from bottom and 3rd from top playing a one off? I remember Charlton perennially escaping relegation in those.
Indeed it was, Charlton finished 19th in the old first division but ended up in the playoffs, the final in those days was 2 leg affair, but was tied on aggregate after both legs, so they had a replay, Charlton won 2-1 after extra time, scoring both goals in the last 7 minutes of extra time.
Chelsea were relegated after losing the playoffs in 88, they won the second division 88/89 with record 99 points for that division
1990 the playoff final switched to Wembley & became a one off match, Swindon won the final but got demoted to 3rd division[reduced to 2nd on appeal] over financial & betting irregularities, Sunderland were promoted instead.
https://en.wikipedia.org/wiki/English_Football_League_play-offs