Same thing with the iCloud email addresses you can't delete and that have to exist if you have an AppleID, despite never using them.

Notably this doesn't actually prevent a zero-click exploit from being received.

I don't want the huge number of inconveniences of lockdown mode. I just want to have apple block messages from unknown senders to iMessage server-side, before they forward any untrusted data to any of my devices.

lockdown mode also doesn't add server-side filtering for unknown imessage senders, so that doesn't seem as good for this purpose.