I didn't understand why it's so important for it to be Linux, as opposed to any other operating system that's as hackable/customizable. So I wanted to know if I had missed something.
I should apologize for my response then. I'd thought you were alluding to iOS, and thought that it was strange to bring that up when discussing configurable OS's.
As it is now, the PinePhone is officially supported by Manjaro and that makes it the most obvious option. Also, all attempts thus far to bring open source to mobile has been some form of Linux, so it's natural that when people think "open source phone", it's probably going to be using Linux. PinePhone itself already has 18-ish different distros, including ports of Debian and Ubuntu, so it's certainly well on its way.