The feature-flags tag appears to be about Feature Flag Toggles, also called Feature Toggles.
I suggest to call a cat a cat, and rename this tag to feature-toggles
I don't think the tag has to be renamed nor to be set as synonym. There could be valid questions about feature flags (formalism, documentation about them, process of definition), which may not be related to feature toggles but to feature ramp up which carry a less binary meaning.
Of course this is terminology only, it's just to ease in a classification goal more than objective definition.
So I'd leave those tags as is, and tag edit the questions which need it.
For generic purposes: If you think questions are wrongly tagged, edit them. If there's too much to edit, then open a meta post to coordinate efforts.
They could also both exist, as synonym tags: What are tag synonyms and merged tags? How do they work?
If we have to choose one of them (as the main and/or only tag) my vote goes with your feature-toggles proposal as IMHO it indeed represents the concept better.
Tags are used to show related questions on the left side. I use tags to set tag favorites and search for questions. You can also let SE sent you a weekly newsletter based on your tags. Also tags are useful for searching for something.
If the tags are too specific, they do not have any use. Try to come up with one tag that covers the whole area.