Hi, i just happened to find this out - thankfully before I arrived in Europe. Passing it on to help others.
I sent an email to a popular hotel in Italy with this travel forum because I wanted to use my 2020 voucher. I didn’t hear from them for a couple of weeks until receiving an email yesterday. In the meantime, I really wanted to secure a reservation for one night at that hotel, so I went ahead and reserved a refundable room through Booking.com for a night in August. I assumed if I heard back, I would cancel the Booking one. If not, I still wanted to stay there and would pay for the one night.
In their email, they said they have not been open since November, and they are waiting to see if they get enough reservations to justify opening in August. Currently they are not at that quantity.
The clue that I am now checking with all of my hotels are the reviews on Booking.com. If I see reviews from June/July, I know they’re actually open. When I checked this hotel, the newest review was in 2020! Hope this prevents one of you from having an unexpected bad experience. 😉