You need to be actively deleting them after the feature has gone live

i could see it getting complex with a lot of flags, inevitable you'll run into a situation where more than one flag is combined.

something like enabledFeature = (flag1 || flag2 || flag3) && flag4

then, down the road, you remove flag4. Hopefully the above would result in a compile error but you may be in a language or situation where a missing flag4 is interpreted as boolean false. That could cause all kinds of havoc in logical combinations like that are scattered all over the codebase. Even worse would be REST APIs retrieving flag values because who knows what's happening in that service code? Plus, you'd never know there was an issue until users start reporting missing or extra features showing up unless you have e2e tests for every possible combination of feature flags...

Create story to add feature flag controlling access to the feature.

Immediately create story to remove said feature flag controlling access to the feature and review it during backlog refinements.