In the documentation here: https://netxms.org/documentation/adminguide/network-discovery.html "A zone value of 0 matches all zones."

However, I actually have a zone with UIN 0 (first zone I created) and the subnet exclusion automatically assigns to that zone when I save it. How do I create an exclusion that applies to all zones in this case?

Please advise, thank you!

This is logical flaw in exclusion processing. I've registered an issue: https://github.com/netxms/netxms/issues/3349
Will be fixed as part of 6.2.0 release.

Best regards,
Victor