Talk:Eternal (format)
Eternal vs non-rotating
This article, and I'm sure others, implies Eternal DOES include non-rotating formats beyond the Forever Four that are listed: https://magic.wizards.com/en/articles/archive/magic-digital/mtg-arena-state-game-kamigawa-neon-dynasty-2022-02-09
Specifically, it ponders "how we'll support an additional Eternal format" due to changes in Historic, which implies that both Historic and the hypothetical new format (which turned out to be Explorer) are indeed Eternal. Official sources certainly make mistakes sometimes, but with some issues WotC staff just goes back and forth on them, and at that point it's not justified to insist on one version just to maintain our preferred conception of it. I don't actually know whether this is one of those issues, just speculating. (I do notice that there's no reference for the clarification that non-rotating formats are not Eternal, however.)
Second, if we maintain that other non-rotating formats aren't Eternal, I don't think there's a page or term for those formats, which is frustrating. Personally I prefer the definition of Eternal == non-rotating, at which point you can further subdivide it based on what the earliest included set is, Alpha simply being the earliest possible one, but in any case there should definitely be a page where we can actually list them rather than various "Modern plus some other stuff" -style awkward phrasings. I might work on that sometime, but only after I try to find out a bit more about which way official sources support.