I think it’s up to the writer. However, adding things to history can break it.
Personally, I prefer to be limited and consistent about what I make up. In my War of the Roses series, nobody above the rank of minor lord is fictional. Nor do I change any events. I only add stuff that might have missed being recorded.
I feel OK about making up petty lords and commoners because the stories are about the Wars of the Roses from the point of view of the gentry in general, not a specific family.
If I had a visiting aristocrat from – say – Germany, I would probably just make him up. The stories aren’t about German history.