Should be SHA256
$ sha256sum check-mk-raw-2.1.0p4_0.bullseye_amd64.deb
a83236c449704e668485e94f7b67fa8dfd6235053616dc93cbee30bd865592e1 check-mk-raw-2.1.0p4_0.bullseye_amd64.deb
But your question is totally valid. This should definitely be mentioned on the page (as already requested in a separate topic: Neue Download-Seite - #16 by martin.schwarz)
No need to have users guessing …