Samsung released One UI 2.5 last year but will not continue with that pattern this year; instead eschewing One UI 3.5 in favor of One UI 3.1.1, with the upcoming skin version set to debut on the Galaxy Z Fold 3 in August. One UI 4.0 is also on the way, as expected.
https://www.notebookcheck.net/No-One-UI-3-5-One-UI-3-1-1-to-debut-on-the-Galaxy-Z-Fold-3-One-UI-4-0-set-to-begin-Galaxy-S21-testing.547392.0.html