Is it reasonable to ask for access to an advisor/supervisor during summer time?
Supervisors are employees just like everyone else. They have duties (to supervise you), and they have vacations and business trips. Both vacations and business trips (conferences, field trips, probably archive work in your line) will be more common during summer. Specifically for academia, many people will use summer to spend more time working from home while they don't have to give lectures, but they may designate certain days when they are in the office, specifically to meet with everyone on a single day.

That said, I see no reason whatsoever why email contact should decrease during summer, of course except for known vacation or trip periods, where everyone can agree that emails may take a little longer to be answered.

Face-to-face contact is also important, but given that many people will spend more time at home, there is a case to be made that you may not meet quite as frequently with your advisor during summer, even after accounting for vacations/trips.

Nevertheless, I see no reason why a request for email contact and (possibly somewhat less frequent) face-to-face meetings should be unreasonable. It would of course be optimal to work out a schedule before summer, taking vacations etc. into account. "I'll check in every day with a short email, and I understand if you reply later to email questions. Could we set up at least some face-to-face meetings, perhaps not every week as during the semester, but every other week, except for the week you are at a conference?"

After all, your supervisor will be just as interested in hearing from you and knowing that you don't spend all of summer at the beach...