Does anyone know what this new “System Customization” in System Services does? It’s constantly using my location and draining the hell out of my X’s battery. Can I turn it off without any adverse effects?
I doubt this feature is draining your battery. What's more likely is that you've recently updated to iOS 13 and your iPhone is still going through its post-update procedures (re-indexing stuff, for one).
I'd leave the option on and wait a couple of days.
"System Customization: Your iPhone will customize system appearance, behaviors, and settings using your current location, for example by automatically enabling Smart Charging when you are at home