The ssh timeout functionality was removed in the OpenSSH-server-8.4
, Now this functionality has been fixed(restored) in OpenSSH-server-9.2 (refer) released in Debian bookworm. The functionality was working properly in Buster and as per the fix it is restored in bookworm. It would be great if we can backport the fix to Debian bullseye as well. A security point of view functionality has high importance.