I could not locate any official announcement but something has definitely changed.
The images that loaded in the second generation photo viewer had the context menu obfuscated. Google made it impossible to download the photo on your computer from the context menu.
The original photo viewer at the time of launch was much simpler and made it easy for the user to download the photo. This option is now available again!
It is not clear what changed Google’s mind. I assume that it was a good thing that the images were harder to download for random people visiting your posts on Plus. People with access to the photos still could download them using the Options menu at the bottom (which provides access to the full size if permitted).