This issue will be solved in the upcoming DSpace 8.0 release. As of 8.0, DSpace no longer sets the "dc.date.available" field by default (existing values in this field will still remain). The reason this has been removed is that the "dc.date.available" no longer accurately represented the correct date when an embargo existed, as
Embargoes are now stored on the Item's resource policy (and not in metadata).