This source cautioned though that the S25 Slim might only be released in limited quantities or regions, so Samsung can judge how much interest there is in it before committing to

The Galaxy S24 FE that could be thought of as the Slim's predecessor is 8 mm thin, which isn't as thin as the 7.2mm Galaxy S25 that just debuted. One rumor suggests that the goal would be to approximate the Galaxy Z Fold 6, which is 5.6mm thin when unfolded, but another number being floated around is 6.4mm.