The flags are currently stored as static vector inside a private getter:

namespace {
std::vector<IncrementalRolloutFeatureFlag*>& getMutableAllIncrementalRolloutFeatureFlags() {
    static StaticImmortal<std::vector<IncrementalRolloutFeatureFlag*>> flags;
    return *flags;
}
}  // namespace

All functions inside IncrementalRolloutFeatureFlag that need to access all flags (like IncrementalRolloutFeatureFlag::findByName()) do this by calling getMutableAllIncrementalRolloutFeatureFlags().

As follow-up from SERVER-115426, there was this discussion:

Shouldn't these live on the ServiceContext rather being statics like this? I don't think we need FCV before we have a ServiceContext do we?

So: Where should IncrementalRolloutFeatureFlags be stored?