Back in the day, Light Flow was an integral part of the Android enthusiast toolkit. Nexus phones came with snazzy multicolored LEDs for notifications, but customization options were limited. Light Flow filled that gap with per-app and per-notification type tweaks. Sadly for those that might still be using it, Google's API targeting requirements will be limiting colored LED support going forward. Per-contact and call/SMS/MMS distinguishing notification customization will also have to be abandoned as a result of other developer policy changes.
The changes were detailed in a recently published announcement. Light Flow developer Reactle lamented being required to disable functionality as a result of Google's SMS and phone permission policy changes, understanding that customers will probably be upset, so advertisements have been disabled throughout the app "as a softener." However, the older features will still be available via direct app downloads from the developer's site. It also sounds as if the premium, "license version" of the app will continue to have per-contact customization, presumably via some other means.
Sadly, it's the API targeting requirements that will have the biggest impact on Light Flow's most well-known feature. Android 8 Oreo drastically changed how notifications worked, requiring that an app declare its notification LED color and vibration/sound when created, rather than passing that information later. That change limits Light Flow's ability to control LED notification colors, sounds, and vibration patterns, though other functionality is unaffected.
The developer is still trying to determine the best course of action and whether or not it's better to remove the LED functionality to update the API target, or keep the app at its current version targeting Android 7.0. In either case, Reactle assures its customers that it will continue updating the app via its own site — we presume with the LED functionality preserved. Unless Google adds an LED-specific API for developers, that's the best that can be done.