They are supposed to be removed quickly. The biggest issue I've seen with implementations of feature flags in products are that they are not removed quickly enough. They should be removed as quickly as possible, to avoid what you're saying. In some places, I've even advocated and implemented strategies for not considering something actually done until the feature flag has been removed. So it can be deployed and activated in production, but in terms of project management, the overall task of the feature shouldn't be considered done until cleanup has been done. This process has been the most successful.

Another strategy that kind of worked but not as well, is requiring the cleanup of the feature flag to happen in the next sprint. But it tends to be de-prioritized, and sometimes even pushed to the next sprint as other things are always more important.

Another strategy that I've discussed and thought about but never actually implemented or advocated for, is limiting the max amount of active feature flags in the specific project to N numbers (ideally 1). So if you want to activate another feature flag, you have to first cleanup the previous one.