I’d have guessed promotion/relegation started well into the 80s. I always remember Crewe regularly (they may have just been down the bottom) having to apply to retain their place
Disaster struck in 1978, when the club was voted out of the Football League following three consecutive 23rd (out of 24) placed finishes, and was replaced by Wigan Athletic. The voting couldn't have been tighter, as the clubs drew on the first ballot (when many had expected Rochdale to be voted out), but Wigan's superior canvassing ensured that they won the second ballot. Southport was the last club to leave the Football League through the re-election process. Automatic relegation from the Fourth Division was introduced in 1986–87.
you're right!