Apple iPhone 7/7+ (apple-iphone7)
These two devices can boot Android (via Project Sandcastle) so a postmarketOS port could be possible once flash storage drivers are added.
Users owning this device
How to enter flash mode
No proper flash mode, but kernel can be booted via checkra1n/PongoOS.
First you have to enter DFU mode. Power off your phone and connect it to your computer via USB: Hold down+ for 5 seconds (after Apple logo appeared). Release and keep holding for another 10 seconds.
To enter PongoOS, from where we can load the Linux kernel, run follwoing command on the host machine:
$ ./checkra1n-cli -cpE
PongoOS will start booting and printing messages on the screen.
There is a manual method described in this blog post.