Has an AFL game ever ended 23-21?
Exactly once. On May 4, 1901, Geelong beat Melbourne 23-21, and no AFL game before or since has ended that way. When it happened it was a scorigami, the first 23-21 in league history.
That makes 23-21 the 3836th most common final score out of 6,607 that have ever occurred in AFL. One point away, 24-21 has happened never and 23-22 has happened never.