Datalog may appear to be a subset, but it is quite distinct semantically.

What is Datalog used for nowadays?

Datalog is not a subset of Prolog. It looks that way because both are based on Horn clause logic, while Prolog is more expressive.

This loops Prolog, but terminates in Datalog:

p :- p.

p.

?- p.

This is because the underlying mechanism is completely different. Datalog is like SQL with recursion, you start with known facts and repeatedly applies rules to derive all consequences until nothing new appears. In Prolog, you start from the query and works backward through rules until it either finds a proof or fails.

So, Datalog treats Horn clauses as database constraints/inference rules while Prolog treats Horn clauses as a search program. They use the same mathematical substrate, but completely different computational models.