When a retention tag is removed from the retention policy applied to a mailbox, the tag is no longer available to the user and can't be applied to items in the mailbox. Existing items that have been stamped with that tag continue to be processed by the Managed Folder Assistant based on those settings and any retention action specified in the tag is applied to those messages.
However, if you delete the tag, the tag definition stored in Active Directory is removed. This causes the Managed Folder Assistant to process all items in a mailbox and restamp the ones that have the removed tag applied. Depending on the number of mailboxes and messages, this process may significantly consume resources on all Mailbox servers that contain mailboxes with retention policies that include the removed tag.
https://learn.microsoft.com/en-us/exchange/security-and-compliance/messaging-records- management/retention-tags-and-policies#removing-or-deleting-a-retention-tag-from-a-retention- policy