“My Photo Stream” was just a sub-set “Album” of iCloud Photos for short-term photo storage of recent photos until they were synced with your other iOS devices/Macs, etc. that didn’t count against your 5GB of free Apple iCloud Photos storage. Now your new photos will immediately go against the 5GB free limit...methinks it is a “revenue enhancer” to sell more iCloud upgraded storage plans starting at 99 cents per month. ;-)
See:
https://www.apple.com/icloud/
Actually, no, it wasn’t a sub-set “Album” but instead it was an early iteration of the concept existing solely in the Cloud before Apple developed the Shared Album concept. A user would upload pictures to the “My Photo Stream” location unique to that user, then give access to that location to other users whom he/she wished to share that site with. They could then see and download the photos there, and if, given permissions, upload photos for other users, or just for the owner of the account to see. It was awkward to use and navigate.
I set one up and found it so, and soon deleted it, opting to just send and receive pictures to family members via email. Too much time was spent trying to tell family members how to do things on “My Photo Streaming” and others who had their own accounts had trouble coordinating with others.
Apple had expanded it from the system in place that allowed all devices in one AppleID user’s account to share documents, photos, etc., but that did not lend itself well to sharing just photos with other AppleIDs and certainly not with non-AppleID accounts which they tried to Jerry-rig onto it.
It simply was not ready for prime-time.