The element no longer accepts a scheme attribute, it’s now obsolete and should be removed.
For example, old documents adhering to old definitions in DCMI (Dublin Core Metadata Initiative) use this HTML tag to define a date:
As the scheme attribute is now obsolete, it should now be removed. The following HTML code will pass current validations, but you should check the exact definition to use if you want to keep using the DCMI tags.
Learn more:
Dublin Core Metadata Initiative