I have been trying to delete a smart mailbox for over a year

I’m trying to permanently delete a Smart Mailbox that’s stored in iCloud Mail, but I can’t get rid of it.

Here’s what I’ve already tried:

• Deleted the Smart Mailbox from the Mail app on my Mac.
• Confirmed that no rule or filter exists that’s recreating it.
• Searched for and couldn’t find any SmartMailboxes.plist file on my Mac
• Turned off iCloud Mail syncing on both my MacBook and iPhone. Mailbox reappears in iCloud.com in a browser.
• The Smart Mailbox keeps reappearing whenever a new email is received.

It seems to be syncing from somewhere in iCloud, but I can’t find any way to remove it from the server side.

Has anyone found a way to permanently delete an iCloud Smart Mailbox (especially if it doesn’t show up in Mail rules or .plist files)?

Any help would be appreciated!