How to delete an old iCloud account from an iPhone if the Apple ID has been deleted
This article looks like it may be your answer.
It details how to deactivate your old AppleID in particular.
You could also, if I understand your problem, put your AppleID back on the iPhone, turn off Find my phone, then again delete the appleID.
Or check this link for ways to do it without having your iPhone set up again.

https://www.macrumors.com/how-to/how-to-delete-or-deactivate-your-apple-id-account/