Filler episodes in my eyes are ones that are completely forgettable, i.e. after a while you barely remember the plot because it is just meh. On the other side there are episodes like In the Pale Moonlight ( DS9) that are still talked about today.
Yes, as I said, the term should be referred to the most run-of-the-mill standalones, rather than the way people tend to use it today, as any episode that doesn't advance an ongoing arc.
I will grant that 26-episode seasons are too long, because there isn't time to make every episode as good as it can be, and sometimes you have to settle for a mediocre episode just to meet the schedule. So maybe something like 13-20 episodes is better for having a good mix of episodic and arc installments.
Then there are bottleneck episodes - cut together with scenes from previous episodes to save costs, usually if the season went overboard with the budget or they need to save money for a big mid or season finale, where they go all in. These are the worst examples of what a 26 episode full season had ( Star Trek had some of these but the worst culprit was Stargate, that had such episodes on a regular basis).
Those are called clip shows. You're thinking of bottle episodes, which are episodes filmed only on standing sets to save money (which can be superb, e.g. TNG's "The Drumhead" or DS9's "Duet").
Many series, like
Stargate SG-1, have annual clip shows built into their budgets from the start. It used to be that they were always pretty routine, disposable stuff, like TNG's "Shades of Gray," but starting in the '90s, shows that had to do clip episodes occasionally put more of an effort into making them creative and worthwhile. SG-1 was pretty good at this, with clip episodes like "Disclosure" or "Inauguration" that contributed meaningfully to the story arc.
Hercules and
Xena had enormous fun with their clip shows, often framing them with the cast playing new characters in the present day -- the best being "Yes, Virginia, There Is a Hercules," in which the show's cast played comedy caricatures of the show's own writing staff, with the conceit being that the actual Hercules was pretending to be an actor named Kevin Sorbo playing Hercules. (A conceit that really does not hold up in retrospect.) The '88 syndicated
Superboy had some really lame clip shows, but in season 4 it did "Who is Superboy?," which is one of the best episodes of the entire series despite being a clip show, because instead of just using the plot as an excuse to set up clips, it used the clips as a vehicle for doing its deepest dive ever into Clark Kent & Lana Lang's relationship.
Andromeda's "The Unconquerable Man" is one of the best clip shows ever, since it incorporated the flashback clips into an alternate timeline that cast the entire previous series in a new light.
Really, if you think about it, TOS: "The Menagerie" was a clip show. Its story was built entirely around clips from a earlier episode in order to get the show out of a budget/schedule crunch. It just happened to be an unaired episode. So even back then, clip shows could occasionally be worthwhile, though that was more exceptional then. (I tend to think "Shades of Gray" was so bad that it lit a fire under TV producers to try to make better clip shows from then on, or just to avoid them altogether.)
I always found it weird that
Stargate SG-1's sister production, Showtime's
Outer Limits, had annual clip shows built into its budget like most Canadian-filmed shows of that era. After all, it was an anthology. It's weird to take a number of separate, self-contained stories and try to knit them into a shared universe. Although in some seasons, they set up deliberate continuity links between episodes (e.g. the same robotics company being involved in all of them) to plan ahead for the clip episodes.
These days, the only US production I can think of that still does annual clip shows is
Power Rangers, which tends to do them as its annual Halloween and Christmas episodes. Which is weird, because the entire franchise is a clip show, built around recycling action footage from Japan's
Super Sentai. (Which has its own tradition of annual clip shows, hard to avoid since it tends to have around 48-50 episodes per year.)
