However, Apple seems to do a decent job at isolating elements from third party app icons and replacing their background with the dark gray gradient, as seen below:
View attachment 2388170
So, while I do agree that Apple is expecting developers to update their own icons with dark mode variants, I don't think it would be hard for Apple to "force" a dark mode on some icons the way it can "force" tinted icons.
For example, the August app and the ParkMobile app are solid colors with a white logo. I don't think it's impossible for the same system tinting icons to replace the background (which it can already do) and just put a red gradient filter over the August logo, or a green gradient filter over the ParkMobile logo.
Or for another example, ChatGPT, Threads, Mapper, and ProCam are all already black icons. They could easily replace the black background with a matching dark gradient, or conversely force a "light mode" icon by just inverting them.
Obviously, it wouldn't work for every app, and also the system doesn't just inherently know "oh this icon should be considered the "light mode" one, let me generate a "dark mode" one, or vice versa. However, I do not think it's impossible. The tinted icons already prove Apple can *mostly* detect icon elements, separate and tint them, and do background replacement. The only difference between the tinted icons and and a forced dark mode icon would be applying a matching color gradient to the original icon's colors, instead of a solid user chosen one.