The tricky requirement that ends up existing and torpedoing attempts to clean up feature flags is a requirement for long-term holdback.

e.g. "Test was successful so it's rolling out to all users, minus a 0.5% holdback population for the next 2 years"

This then forces the team to maintain the two paths for the long-term, ensuring the team might get re-orged / re-prioritize their projects sometime a year later making the cleanup really hard to eventually enforce.