Flags are excellent at what they are for and terrible as permanent architecture. Ten long-lived boolean flags describe over a thousand possible configurations, and your test suite covers one.

Give every flag a death date

• Set an expiry when you create it, and fail the build when it passes. A noisy build is cheaper than a forgotten branch.
• Separate release flags from operational toggles and permission checks. Only the first kind should expire; the other two are features and should be named as such.
• Removing a flag means deleting the losing branch, not just the condition.

The cleanup is not glamorous work and it never appears on a roadmap, which is exactly why it needs to be automatic rather than intentional.