In the episode just before the series finale (DS9 : Dogs of War) Sisko, Martok, Admiral Ross and Velal mention that the Dominion forces have completely withdrawn into Cardassian space.
INT. WARDROOM : Where Sisko is meeting with MARTOK (wearing his
chancellor's cloak), Ross and the Romulan Commander last seen in "When
it Rains" -- who we'll call VELAL. They're all looking over a wall
ROSS : -- according to our intelligence reports, the Dominion has
completely withdrawn from Klingon, Federation and Romulan space. They
seem to be forming a new defense perimeter within Cardassian
There were some in-universe tactical reasons for this, but most likely it was also a way for the writers to tie up loose ends like liberating Betazed.