With the launch of the Librem 5 phone and the upcoming PinePhone, it looks like in the near future we will have in the market at least a couple of phones compatible with mainline linux kernel and (I think) without requiring any proprietary binary blobs.
Regarding mobile operating systems, the Librem 5 comes with PureOS, which is a Debian Buster derivative that uses the phosh “desktop” environment (you can see it here in action). I’ve tested it in a virtual machine following this instructions and, although there are lots of missing functionalities, it feels like home if you are used to GNOME. It doesn’t currently have lots of apps but flatpak is installed by default.
The PinePhone makers have delegated to the community the operating system creation and the people at PostmarketOS, UbuntuTouch and other projects are working on providing ports for this phone.
In my opinion, Silverblue with the posh desktop environment could be a great mobile OS. OSTree seems like a perfect fit for this kind of devices and the apps could be installed via flatpak.
I think currently there is not arm/aarch64 port of Silverblue and the boot process for phones is different to regular computers so making it possible won’t be easy nor fast. Anyway I wanted to share this idea with you and know your opinion about it.