There appears to be an issue when using Debian 11 where spamassassin won't register to start at boot, and even in a post-install rebooted state, will not be running. Perhaps I am incorrect, but I think this is a useful addition that makes sure this is all set up to operate as you intended. I am not much of a systemd guy, but I think this is the correct way to do this. It appears to work on a server I used this script on. Please reject if superfluous or unnecessary.
Thanks for your work automating the "really fun" part of all this!