Do changes made to photos in My Photo Stream propagate to all the devices sharing that Photo Stream?

How does that jive with the definition that all devices with My Photo Stream share the same photos?

That definition is true for iCloud Photos, but not my Photo Stream.

If you want to have all edits and adjustments in sync across all your devices, you should use iCloud Photos, not my Photo Stream. iCloud Photos is perfect for syncing and keeping the libraries on all your devices identical. Your Photos Library will always be identical on all your synced devices and adjustments you apply will update across all devices. And it is working for all kinds of media in your Photos Library - photos, Live Photos, Portrait Mode photos, videos.

My photo Stream on the other hand is as streaming service for transferring new photos, not a syncing service. It is a temporary buffer in iCloud. When you take new photos on your iPhone or iPad, or import new photos to the library on your Mac, the original photos will be uploaded to My Photo Stream, right as they have been imported from the camera. The filters you applied by the camera may be applied, but no later adjustments you added to the photos in your library. They will not be updated in my Photo Stream, unless you import the adjusted photo again. My Photo Stream is a temporary buffer. Older Photos will be purged from My Photo Stream in iCloud after 30 days, even if you are still seeing them on your devices in My Photo Stream. Locally, on your devices, My Photo Stream is caching up to 1000 photos, what you will change in this cache will not be streamed to iCloud.

Apple has not updated My Photo Stream during the last two years. It seems to be phased out. Users with a new AppleID cannot even activate it. It is only available for long time Apple users. Use My Photo Stream on iPhone - Apple Support

If you recently created your Apple ID, My Photo Stream might not be available. If My Photo Stream isn’t available, use iCloud Photos to keep your photos and videos in iCloud.

With a recent iPhone model, that can take Live Photos or videos, I would not use My Photos Stream. It will make it complicated to transfer any of the newer media format. It will just create a mess of of partially transferred Live Photos by just transferring the still frame and you will have a hard time to download the full versions manually.

It is strange, that you are not seeing a My Photo Stream on your iPhone. the My Photo Stream album will be hidden, if you have iCloud Photos enabled. Are you using iCloud Photos on your iPhone?

I cannot edit my previous post, the first part should read:

That definition is true for iCloud Photos, but not my Photo Stream. (https://support.apple.com/en-gb/guide/iphone/iphbfeb468fc/ios)

Your photos automatically upload when you leave the Camera app and are connected to Wi-Fi, but edits aren’t updated across all devices.

My Photo Stream is only transferring the original versions. It will not update the version in the stream. After editing a photo on your iPhone duplicate it and save it a new photo. Then My Photo Stream will upload the adjusted version to iCloud and sync it to your other devices.

If you want to sync adjustments to your photos across all devices, it would be much easier to use iCloud Photos. iCloud Photos will keep the libraries on all your devices identical and not just transfer newly added photos.

See: https://support.apple.com/en-gb/guide/iphone/iph961b96c4d/ios

Yes, I just checked, and on the iPhone I have iCloud Photos enabled, Optimize iPhone Storage checked, Upload to My Photo Stream enabled, Upload Burst Photos disabled, and Shared Albums enabled. In the app, under Albums, I don't see My Photo Stream under My Albums or Other Albums, incl. Hidden section...

That explains it then, that you are not seeing the My Photo Stream album. It will be automatically disabled, as soon as you turn on iCloud Photos. You cannot use both iCloud services together on the same device.

The other concern I have is that iCloud Photos--as far as I understand--replicates (everywhere the feature is turned on) not only the photos, but also their presence/absence, much like iCloud Drive: if I ever erase a photo from one of the devices participating in iCloud Photos (e.g., to recover space), t

What I am doing to avoid this problem, is to keep the iCloud Photos Library so small, that it will fit seamlessly onto the device with the least storage. You can have several Photos Libraries, and only the one you designate as the System Photo Library will sync with iCloud, if you enable iCloud Photos.

In my iCloud Photos Library are only my favourite photos that I want to see on all devices, as well as the most recently taken photos I am still working with. All other Photos I will move to my archive of older photo libraries on my external archive volume.