PHOTOSTREAM CONFUSION

Yes. However, they should go away in 30 days or if you load more photos in the PhotoStream from your camera above 1000 photos.

This explains it all: Delete photos from My Photo Stream - Apple Support

Frankly, I would never delete photos from My Photo Stream. It is supposed to work as OT described above, see: Delete photos from My Photo Stream - Apple Support, but I have seen occasionally, that photos I imported to Photos on my Mac or on my iPhone have been deleted from the Library, when I deleted them from My Photo Stream. It should not happen, but the behaviour of deleting from My Photo Stream has changed with every system version. Apple seems to be phasing out my Photo Stream anyway. Apple has never updated it to support videos or new image formats like Live Photos and is not suitable for modern iPhones and iPads.

And as the photos downloaded from My Photo Stream on an iPhone or iPad are having a smaller, optimised size, they are a continual source for duplicates on my Mac and irritating. I stopped using my Photo Stream, when I started to use my iPhone as a camera and needed support for Portrait Photos, Live Photos, slow motion videos, and HEIC.

And since more than two years I am seeing this warning in all Photo Stream support documents (My Photo Stream – Apple Support (UK)):

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

Only users with an old Apple ID can still use My Photo Stream at all. It seems The End is Nigh.

So you have a separate library for each year? That's counterproductive to what Photos is designed for. I just create a smart album with these criteria:

For 2021:

Much quicker and easier than creating a whole new library.