Security Admin Center Tenant Allow/Block List Not Able to Block IPv4?

The documentation for the New-TenantAllowBlockListItems cmdlet was updated to reflect that only IPv6 entries are supported with the Entries type of "IP". Additionally, the "Create block sender lists" documentation was updated to say "IPv4 ranges aren't supported yet. Admins can create and manage entries for IPv4 addresses in the default connection filter policy".

I suspect when support for IPv6 addresses was added in CY2024 Q4 that there was an issue with IPv4 addresses that appears to be being worked on according to the wording quoted above (I would provide the links but am not sure if they would pass through in the comments here).

So, if you need to block IPv4 addresses, you can use the Connection filter policy under the anti-spam policies in MDO but be aware that blocks via this mechanism don't get marked as spam (or other Defender features) as they are flat out rejected. I.E. this appears to happen earlier in the SMTP message pipeline process.