It will flower as usual without any pruning, in fact the old stems tend to keep the new softer stems supported. If it is never pruned at all, it will just get a bit big & overgrown, so it's just up to you really to contain it, so a light prune each year is fine.

I left one of mine un pruned for 5 years and it was fine. Then when I did prune it, it didn't seem to mind at all.

I only tried leaving mine unpruned one year to see if they wold flower any earlier but they didn't.

Come to think of it those weren't shrubs though. Shrubs can just be left to get on with it