It's fine for them to exist, and probably better -- you can land incomplete things behind a feature flag and avoid some amount of rebase/merge hell.
But they should be marked experimental in that case, not optional. Optional flags ought to work together, or at least it should be considered a bug worth quickly fixing if they don't. Combining experimental flags otoh is Buyer Beware.
But they should be marked experimental in that case, not optional. Optional flags ought to work together, or at least it should be considered a bug worth quickly fixing if they don't. Combining experimental flags otoh is Buyer Beware.