I think it's better to admit they actually are config, just a different kind of config that comes with an expiration date.
Accepting reality in this way means you'll design a config management system that lets you add feature flags with a required expiration date, and then notifies you when they're still in the system after the deadline.
I agreed. My perspective is that there are two kinds of feature flags: temporary and permanent.
Temporary ones can be used to power experiments or just help you get to GA and then can be removed.
Permanent ones can be configs that serve multiple variations (e.g. values for rate limits), but they can also be simple booleans that manage long term entitlements for customers (like pricing tiers, regional product settings, etc.)
Accepting reality in this way means you'll design a config management system that lets you add feature flags with a required expiration date, and then notifies you when they're still in the system after the deadline.