"Dark mode can be forced for all apps" on a stock Android 11 ROM? I think you're imagining things.
Older stock ROMs only showed installed apps (not system apps) that had a launcher icon.
Later they replaced it with a hardcoded list, which is still the case on the latest stock Beta ROMs based on Android 11, while on our ROMs all non-system apps are displayed.
As for Android 12, an external configuration XML file on system partition was used, and nowadays there are 2 external XML files which further configure which components should be affected by the switch...
Like I said, the feature was not removed... On Android 11, there's a hardcoded list of apps that, if installed, are shown in the list.
On Android 12, however, Xiaomi moved the supported app list handling to MIUI's framework, which loads the supported apps and their configurations from 2 external XML files on system partition, and will later be updated with additional apps from Xiaomi's cloud server. We don't decide the contents of these lists. It just so happens that most of the apps Xiaomi approved as supported are Chinese apps.