Likewise, while the idea of sticking the determination of whether a given feature is enabled or not in a REST service would make sense, it's not at all clear why you'd ever want to farm that out to another provider, apart from the rare case when you're making a static website with no backend at all.

If you have any kind of back-end infrastructure, it'd seem trivial to implement this yourself. Doing it yourself also allows you to make the feature flags more controlled, e.g. by some flag associated with the current user such as opting into experimental features.

For efficiency, I'd also want to batch a bunch of such flags together, so e.g. when a use logs in they get a list of enabled features that gets cached locally, rather than having to query each and every one via REST.

So I'd echo the question, what problem is this solving?