I am an Android user for a long time (since Android 2.2) and used pretty much every version from then on. Google devices (from Nexus to Android One devices and now Pixels) pretty much always allowed unattended updates for a long time (you may be right about Android 12, my memory is fuzzy here). And I never remember having scary warnings for sideloaded apps, sure, Android made it more difficult to install them (by having a permission per app instead a global permission, but I would say this was a very welcome change), but it was never convoluted or difficult.
But yes, non-Google devices make this way more difficult, e.g.: Xiaomi devices actually has a scary warnings and they trigger at each reinstall. Also, they messed up something in the install APIs so you can't update apps unattended, needing to trigger the popup to install at each update.
So yes, in general, this is not the fault of Google but third-party companies.
I think what people upthread are talking about is allowing third-party stores (like F-Droid) to do unattended updates. That has not been possible until recently. Up until Android 12 or so (possibly later), I had to manually approve it any time F-Droid wanted to update an app I'd installed through F-Droid itself.
Unlike with apps installed via the Play Store, which can update them without needing my manual approval.
But yes, non-Google devices make this way more difficult, e.g.: Xiaomi devices actually has a scary warnings and they trigger at each reinstall. Also, they messed up something in the install APIs so you can't update apps unattended, needing to trigger the popup to install at each update.
So yes, in general, this is not the fault of Google but third-party companies.