> Is adding a feature-flag really the same as pushing the feature into the browser immediately? It can easily just be part of a SWE needing the flag in place in order to continue work without impacting anything else, even if that thing never ever launches.

Yes, because that's a such anti-consumer issue. It shouldn't exist in the first place, it should never be merged to master. There's no reason to not keep it on a separate branch if you don't intend to use it.