June 2020 travel and hotel reservation

Given your current experience, it looks like reservation systems will book approximately 6 months in advance or so, so I would wait until around December.

It makes sense to me why a hotel wouldn't want "place holder" bookings so far in advance, especially given the rates at which people change their minds or cancel. It would make it impossible for someone who doesn't book in advance to find a room anywhere if all rooms were pre-booked a year in advance (and a large chunk would not come to fruition due to cancellations or changes to different dates, etc.) Plus hotels may want to revise their rates every year, and those are unknowns right now. There's really no need to book that early unless there are extraordinary circumstances like a very limited lodging inventory, a very unique event going on, unusually small town, etc.

I send an email directly to them and ask when the reservation system will be available. Sometimes they just go ahead and make the reservation, but I always confirm a couple months prior to the date to be sure the reservation was entered.

To add to what Agnes said, if you are going to certain small towns that have major events on certain dates, then booking 10 months ahead isn't unreasonable. I'm thinking of Tarbert in Argyll/Kintyre, which has a regatta and golf tournament in May. For those dates every room for miles around is booked way, way ahead -- including a little cabin on a boat in the harbor. OTOH, if you don't need to be in that town on the date of that big event, then you can adjust your itinerary to avoid the crowd.

I've been looking at Scotland for next June and have been using Expedia to gather info.

As suggested above, It might be worth calling or emailing the specific places you are interested in to find out exactly when they will open them up. Many places do let you book far in advance (I’ve already got reservations for April 2020), but it sounds like the specific places you are interested in do not.