Actually I double checked. The HEIF photo was previously sent into the Photo Stream as a JPEG (I checked the photos in My Photo Stream that are still loading to this day because the images were taken before I installed iOS 15.4).
So while HEIF was never supported in the photo stream, it used to work by the iPhone sending HEIF photos as a JPEG just fine
So it is a bug...that wont ever be fixed.
IIRC, when HEIF was introduced, it was explaned, that if you had older devices (like I have old iphone, ipad AND mac) in your Photo Stream "group", the photo was converted to compatible format for those old devices.
I might also remember wrong and conversion could have been done, even without any need for it.
There might, of course, been a bug in that conversion and that might be the excuse to remove that feature now for good.
This seems to have been work in progress:
Apple hasnu2019t killed My Photo Stream, but itu2019s available to ever fewer iCloud users.
www.macworld.com
Btw, I checked my Photo Stream from my mini2018 running Monterey and suddenly there are new photos from my iP11, which are HEIF. And a lot of older photos from my ipadAir4thG, which are also HEIF.
I'm not sure if these got there after I synced these devices in Finder yesterday.
The whole Photo Stream album has no chronoligal order now, maybe it's time for another rebuild of Photos archive...