[winkelwagen]

Difficult to answer without understanding how you are deploying currently. I do think feature flags can be an anti pattern. I would ask the customers why they want it in the first place. Is it a trust issue? Is it an internal process that they require? Ideally you would want to stay flexible, that would mean not tying your company to their schedule. It is a hard application to deploy multiple times for each customer. Perhaps a more lts approach for those customers might work. Just keep in mind the larger the releases get, the more issues it might cause.