After deleting the 'Unlicensed Group' in Aria Operations under Subscription -> Legacy Licenses -> License Groups, the license group comes back following a cluster restart.
Aria Operations 8.18.x
This is expected behavior, as the Unlicensed Group is a default group that automatically contains objects that become unlicensed due to Aria Operations license overuse.
For better license utilization, consider creating a custom license group in Aria Operations. Please see: "License is Invalid" watermark appearing in Aria Operations.