Good test, XOOOO.
I’m not at all surprised by the result.
However, it does not quite demonstrate what you have concluded.
Email protocols, like many other Internet protocols, are constantly evolving! They are changing, and the Internet committees making the changes are not under the control of any single company.
That’s completely intentional.
Of course, it is up to each individual company to implement the protocols themselves.
(Incidentally, the protocols are usually created in such a way as to be reasonably compatible with systems only recognizing the older protocols, so things don’t suddenly “break”—at least, not in anything like a “catastrophic” way. There will still be potential feature related issues.)
What your test demonstrates is that iOS 12.4.8 is using a slightly older protocol than iOS 14.2. (Where, exactly, the newer protocol was implemented, I cannot be certain, at this time.)
It also demonstrates how the PC email client you are using on your PC handles the two protocol variants differently.
In both protocols—I can guarantee—the photos, and other files, are attached, as attachments, to the email: this is a very old aspect of the email protocols.
The only difference is in how the attached files are rendered—how they are displayed—on any given email client, and how the client chooses to handle any downloading and saving of said attachments.
I’m sorry to hear that the PC email client you are using doesn’t appear to support bulk downloads and saves for the newer protocol.
That behavior is not within Apple’s control.