An entry in a pool in a loot table can have conditions and functions, which are essentially predicates and item modifiers. Conditions have a "reference" option, to use an external predicate file, but functions don't. This seems like an inconsistency.
Comments 2
Judging by the changelog, it looks like this was "fixed" in 23w14a. But it's still a feature request, regardless.
They do already use item modifiers in-format directly, it's just that item modifiers can't reference other item modifiers (no type for it).
While inconsistent, predicate and item modifier options are unrelated to each other. This is more a feature request than a bug.