Based on numerous sources, it seems clear that Proto-Indo-European was
Productively agglutinative with non-root morphemes (and perhaps some specific roots that are also able to act like bound morphemes), and
Derivationally agglutinative with roots in deriving new words.
What I haven't been able to find a clear answer on is whether PIE was productively agglutinative with roots, as is found in many modern Germanic languages (i.e. German). Is this still an unanswered question?