I read it as supporting the iphone X with the A11 chip, just not the gpio buttons.

"This series adds device trees for all A7-A11 SoC based iPhones, iPads, iPod touches and Apple TVs."

"The following is supported on A7-A10: - gpio-keys The buttons on A11 based devices like the iPhone X is a SMC subdevice, and cannot be supported in this way."

An iPhone 8 still has a lot of processing power for headless home server tasks. I use a much weaker ARM dev board as an ssh gateway and Wireguard VPN into my home network and it works just fine. The only thing I'd worry about is leaving the battery on the phone and having it puff up after being trickle charged for months on end.

But if you remove the battery and mod the phone to power it directly from an external power supply you're all set!