> Now if you’re talking episodic TV the answer is harder. If there isn’t an overarching plot, wouldn’t every episode technically be “filler?” I think in that case people do actually use the term as an indicator of quality, with bad episodes labeled as filler so that future watchers can know whether those episodes are skippable

That's my hangup on those type of shows that lack a plot like Family Guy for example. That show does have some continuity but characters don't age at all visibly despite holidays and birthdays coming up frequently (google floating timeline and you see what I mean there), references to older stuff do happen from time to time, etc. These shows thrive on that honestly.