Hi,
you're correct - the behaviour regarding SSH as a debug interface has changed with RM 21.03.2. Beginning with this release, the SSH service is disabled by default and automatically enabled when a system-user account is activated. When the system-user account is present, SSH behaves exactly as before - you can assign the permission to other users via the membership in the "sshuser" group. When the system-user account expires or is deleted manually, SSH is disabled automatically.
This change was required to address a security risk which might allow an attacker to escalate its privileges via a combination of SSH access and the Ubuntu Core snap mechanism. Since SSH is considered a development interface, we assume that for regular operation SSH is not required. Therefore, I can currently not promise you that the old behaviour will return. Could you explain your use case? This might help us to better understand who uses SSH and what for.
