The main difference between the elided form and the full one is that the former doesn't carry a tonic accent and the whole group is pronounced as if it were a single word; this applies also to truncation.
The hypothetical sentence
Non ha comprato nessun vestito nuovo questo anno.
would sound quite strange; a case where questo anno could be used is
In questo anno ho avuto difficoltà, ma penso di poterle superare
where emphasis is on questo, because the missing elision forces to place a tonic accent on the word. On the other hand, a better form for the sentence would be
Quest’anno ho avuto difficoltà, ma penso di poterle superare
and here questo anno would certainly cause eyebrow raising. The meaning of the two sentences is not exactly the same, but just by a nuance.
I'd not use questo autunno in the sentence you report, but context would help: if the sentence is followed by something in opposition, then emphasis might very well go on questo.
Note that in several cases the elision is not optional. For instance, “lo amico” is never used in current Italian (it used to be in ancient times) and it would seem talking like Vittorio Gassmann in L'armata Brancaleone, where a mock medieval language was spoken.
Elisions that were taught until rather recent times were gl’inglesi or l’estati, but they have been unused for several decades in spoken Italian.