This is still an issue today!
There's no "unselect all" button in the photos tab
If I've selected multiple photos from different months, I have to scroll to each month and unselect by month/year or just unselect them one by one.
OR refresh page and wait for all photos to load again (this is what I do now)
-Even having to scroll up to the year selector and unchecking there, then trying to find the place where you left off is frustrating when you have lot's of photos.
I would fully advocate for a unselect all button. (See screenshot for the idea)
Seems like a simple thing but would make a huge difference in my life.