If you wish to delete content types that are published from the content type for use by different site collections, should not just be deleted from the content type hub. If this is done, then the delete option won’t be available on the different site collections. An unpublish must be carried out, then time allowed for the site collections to pickup this change. Then the site content type will become local and available to delete.
If the content type has been deleted without the unpublish action and you cannot delete you will need to recreate that site column in the content type hub with the original GUID. To obtain the GUID, refer to the URL in the site collection that inherited the contenttype and copy it ready for the below PowerShell. The PowerShell is as follows;
Connect-PnPOnline -Url https://tenantname-admin.sharepoint.com
Add-PnPContentType -Name “OriginalContentTypeName” -Description “Description of Content Type” -Group “.OriginalGroup” -ContentTypeId “0x01000844CED46FF6524FB95228D0799CD485”