Introduce an alias for the table variable and use that in the expression:
select KeyTerm ,D.Splited from @LeftSuper ls
outer apply [Split](ls.Data,300) as D
This is actually fairly common - because tables may appear more than once in a query, each time that @LeftSuper
is encountered in the query, it's treated as a new reference to the table - not as the reference that has already been added - which the alias allows you to reference.