That is tech supports script, its actually accurate as well. If it not being updated for years wasn't a hint you can goto Apple.com support pages you'll find things like this....
With My Photo Stream, you can access recent photos that you took with your iPhone, iPad, or iPod touch, or that you uploaded from your Mac or PC.
support.apple.com
iCloud Photos and My Photo Stream can help you get your photos on your Apple devices and on your PC. What’s the difference between them? Learn more about both features.
support.apple.com
The same applies if you've haven't turned it on through several updates. For example, my iPhone 11 Pro Max does not have My Photo Stream. Nor do my Macs. I never used it now I don't have the option available anymore I guess (haven't looked too far into it)...
You are "grandfathered" into My PhotoStream.
The reason is, for the last year maybe two Apple has been moving everything over to iCloud Photos and phasing out My PhotoStream. Users with new AppleID for about 2 years haven't had the option for My Photostream. Its limitations that hinder camera capture tech and application of photo degradation are so blatant its embarrassing for Apple to advertise/market the service. No one is taking it away, its just no longer an option to new users...
The only benefit to My PhotoStream is it doesn't use your iCloud storage. But thats it...
Mac and PC will get the original quality but thats not really a benefit if the alternative, iCloud Photos does that across the board.
30 day 1000 photo limit with My PhotoStream.
Due to the lossy "optimized quality" when shared to other iOS device there is obvious photo quality degradation. On top of that if its a newer iPhone capturing in HEIC it will need to transcode to JPG which is lossy conversion plus results in inflated file size and/or potentially further reduction in quality (Apple tends to just use bigger files vs lowering quality). While its not a "data hog" its an inefficient use of data, storage space and processing power thus battery which could be considered a "hog".
So not only is it limited to JPG, TIFF, PNG and some RAW formats, none of which a new iPhone natively captures in without diving into settings (again requiring lossy conversion from HEIC) it also doesn't support live photos, portrait mode, and more importantly videos. Videos are a huge segment of Apples marketing campaign, Apple designs the Mac specifically around the requirements of video editing. (admittedly video capture not being in My PHOTOstream shouldn't be a surprise).
There is no sync function. So if you edit a photo other devices will have pre edited versions. This also means metadata used for Apple servers is stifled, like Faces or adding keywords to certain photos for easy search. Combine that with 30 day and 1000 photo limit results different devices having different photos. These are problems requiring your average consumer to navigate a convoluted process which ultimately can result in the loss of very valuable irreplaceable data (photos).
Not using your iCloud storage is the very least they could do with a service like that.
Don't get me wrong, this isn't a personal attack on your preferences. I don't know the reason you use My PhotoStream and they could be very good and valid reasons. However its not going to be around forever and its not doing anyone any favors either. Think about it this way, Apple is focused on getting the best quality photo and video from very limited hardware (tiny lens and sensor on a handheld device) via machine learning and AI, then they have software (My PhotoStream) that will reduce its quality to provide a quasi automated syncing processes when they have a service that does this losslessly?
All that said, if your GF has an old iOS device she is logged into that has My PhotoStream still available she can turn it on, dump some photos into it and it might carry over to the Photo app on the new iPhone. I don't know if she'll get a toggle to use but its worth a shot if its really important to you and/or her.
Good luck!