The S21 service runs at a irregular frequency between December 2023 and March 2024 aswell as October 2024 to December. During the time where the S21 doesn't run, the S42 that usually runs from Hasle-Rüegsau to Konolfingen is extended to Thun. 
In the future, the S21 will run with a hourly frequency, and the S43 will be replaced by the S42 which will run all year long between Hasle-Rüegsau and Thun.
https://www.bls.ch/de/unternehmen/medien/medienmitteilungen/2024/05-21-mm-fahrplanentwurf
https://www.öv-info.ch/sites/default/files/fap/2024/pdf/340.pdf