I agree that "it was" is possibly best when talking about a summer that is now in the past, but actually, if the summer just got over, I still might use "it's been." Maybe that's not logical, but I still might use it.
"The summer is gone" - I don't think this is as clear-cut a statement as "My summer holidays are over".
I can't speak for everyone, but in my book "summer" doesn't have a definite end. It just shades into autumn, and depends on the weather rather than on the calendar. For that reason, I agree with JustKate. (Nice to see you again, JK.)
I can see that someone who has just returned from their summer holidays, and especially if the weather has turned chilly, would prefer to regard "summer" as something belonging to the past and would prefer to say "It was a fantastic summer".
The present perfect tense is used of periods in the past continuing to the present. But the end of summer is not precisely defined; and nor is the "present". For this purpose, the "present" can include a time days or weeks ago.