I've been thinking about this topic for some time and love that someone else came to the same conclusion!

There are a lot more differences in the requirements for these use cases though, e.g., feature flags products want for more analytics, controlled rollout, etc.; authorization, not so much. Authorization requires access to lots of shared application data/state; feature flags, not so much.

Another area of overlap I see is entitlements/billing, e.g., can this person access this feature based on the plan they've signed up for? I've seen a lot of companies use the same underlying systems for feature flags, authorization, and entitlements (esp at earlier-stage companies and ones with a heavy DIY bent). Is this consistent with what others see/do?