What was the first one-room game?

Two Heads of the Coin (1979) is a one-room game.

https://bluerenga.wordpress.com/tag/two-heads-of-the-coin/

Local Call for Death (also 1979) is a two-room game.

https://bluerenga.wordpress.com/2016/08/04/local-call-for-death-1979/

You could argue the former is actually a zero-room game (the room is described but it’s otherwise all conversation), but if you count that way, Local Call for Death would be a one-room game (since there’s only one room that you interact with).