> Theoretically, we'd be able to download some generic OS image and load them onto devices like we can with Linux ISOs and x86 laptops whenever manufacturers decide to stop supporting older devices.
That’s exactly how GNU/Linux smartphones Librem 5 and Pinephone work.
> Do those phones have UEFI and ACPI support, or SBSA[2] support? If so, can you point me to where I can read more about it?
This question is beyond my knowledge, but you can already install >17 operating systems on Pinephone (https://xnux.eu/p-boot-demo/) and >3 on Librem 5 (PureOS, Mobian, postmarketOS [1]), so it should be there.
It doesn’t have UEFI–it uses u-boot instead. To test whether it has ACPI support, someone who has the phone should do this:
sudo apt install acpi
acpi -V
To determine its level of SBSA would take hours of reading the spec and testing each thing in the spec, and I doubt that very many people care about SBSA.
That’s exactly how GNU/Linux smartphones Librem 5 and Pinephone work.