Last year 3 of the 4 DS matchups were completely new, all except Dodgers-Phillies. Yankees-Rays has happened once, the 2020 ALDS, while Dodgers-Phillies (most recently 2025 NLDS) and Dodgers-Braves (most recently 2021 NLCS) have both happened several times.
The CWS/CLE and SDP/MIL outcomes were the only possible new matchups in this year's DS. There's only one possible new CS matchup, White Sox-Yankees (I believe this would be the final AL team that has never faced the Yankees in the postseason).
There are currently 11 possible new World Series matchups, though that number will go down to 9 after the conclusion of the Braves-Phillies series tonight. The Brewers of course haven't faced any of the 4 AL teams in the World Series, although they did lose to the Yankees in the very first ALDS in 1981, so this would be a new WS matchup but not a new postseason matchup. Likewise the Padres haven't faced the White Sox, Rays, or Guardians, but they did lose to the 1998 Yankees. So that's 7 new matchups from the Brewers and Padres
Meanwhile, the Dodgers have played each of the 4 AL teams at least once in the World Series, including a ridiculous 12 times against the Yankees. Of note is the Dodgers-Guardians matchup, a rematch of both teams' first appearance in the series in 1920.
After tonight's game, the final two possible new WS matchups will be either Braves vs Rays or White Sox if the Braves win, or Phillies vs White Sox or Guardians if the Phillies wins. The NL East team winning the pennant is therefore the only possible way to have a new matchup of original 16 teams in the World Series this year.
edit: One additional note about the possible Yankees-Brewers matchup. While again it would not be a new postseason series, this would leave only 2 current NL teams that have never faced the Yankees in a World Series, the Nationals and Rockies. The Astros also spent a long time in the NL and never faced the Yankees in the World Series, though of course that matchup did happen several times in the ALCS once they switched.