When an Interface is deleted from NFA, it will mark any Alerts configured with those interfaces as "Unknown Interfaces".
If you want to cleanup the Alerts page, in order to get rid of the "Unknown Interfaces" you can run the command below on the NFA console:
mysql -D reporter -t -e "delete from trap_definitions where interfaceGroupid !=0 and interfaceid not in (select id from agents_all_view);"
If you want to get a list of recently deleted interfaces you can run the command below on the NFA console to generate a text file with a list of recently deleted interfaces:
mysql -D reporter -t -e "select * from agent_definitions_deleted;" > C:\deleted_interfaces.txt