Do I need a newer Mac if I want to continue to use Xcode with my iPhone? Recently I updated my iPhone 6s to iOS 12.2.

When I went to compile my app, Xcode told me it needed to be upgraded to deploy to device running iOS 12.2.

When I went to upgrade Xcode, it told me my OS needed to be upgraded to macOS Mojave.

When I went to upgrade my Mid 2010 MacBook Pro to Mojave, App Store said “Nah uh, your computer is too old”.

I tried to restore my iPhone to 12.1.4 from the .ipsw, and iTunes says Apple says “Uh um, nope; I decline to authorize…”

Am I separated from my app and data until I pony up for a newer Mac?
Your computer is too old to be officially supported on macOS Mojave, I'm afraid. Also it is not possible to downgrade an iPhone to an older iOS version, unless you have saved certain information before you did the upgrade (which you didn't do).

However there are two unsupported methods of running macOS Mojave anyways, which will enable you to install Xcode 10.2 and develop for iOS 12.

The first option is to use the Mojave Patcher third party utility found here:

http://dosdude1.com/mojave/ 

It essentially patches a Mojave installation package to be able to run on older, otherwise unsupported hardware.

The other option is to use a virtualisation system such as VMware Fusion or Parallels Desktop and run Mojave inside a virtual machine.