The laws are surprisingly sparse, and all I could find on this appreciably cursory glance is that the “goalkeeper is subject to the same restrictions as outfield players when outside of his box“ which is of course pretty intuitive.
There is absolutely no mandatory send off law, as some in here have imagined. Whingit is right to say that Drysdale must have ruled denial of a goal-scoring opportunity, but a) Eastwood was mistaken, not cynical, the ball was moving towards him and away from Taylor, and Taylor was hardly most likely of the two to win the contest. b) if Drysdale is so keen to punish denial of a goal-scoring opportunity with a red card, then he tripped himself up 15 minutes later by failing to do so.
To me it all smacks of a decision that Drysdale could justify, but really didn’t NEED to make; and no-one likes a pedant with a whistle.
IFAB Laws of the Game
www.thefa.com