Is LiveDisplay (in LineageOS) a reasonable substitute?
I've recently installed LineageOS 13 (equivalent to stock 6.0.1) on my rather elderly Galaxy Nexus. f.lux still doesn't work on it - I was disappointed, but not surprised. Yesterday however I was prompted by a notification to try the built-in LiveDisplay feature, which appears to have the same basic functionality as f.lux, including being able to choose colour temperatures for day and night.
It certainly looks to be doing on my GNex what f.lux does on my Win10 box - do you know whether it in fact has the same sleep health benefits (ie reduces blue light)?