Part of Apple's game is to make updates of core software like its version of office "motivate" either os updates or new hardware purchases. One of the ways to "force" a new purchase is this one. Best I can tell there is NOTHING in new versions of Pages that requires new hardware and/or even os updates. But by making it so you have to "keep up," you are pressed to update or buy new.
Feature updates to Pages is often a few nice touches. I think the main driver of such updates is this "planned obsolescence" approach. Those who try to squeeze a few more years out of an aging Mac will quickly crash into the inability to open files like these. That Mac may be plenty powerful enough to edit word processing or simple page layout Pages files but is simply locked out of such files by (Apple) choice.
The suggestion offered in Post #2 is the only real option and it is precarious if you make a document "beautiful." Keep it very simple and #2 works. But as the document gets visually complicated with lots of design, odds in exporting intact to Word and then back into an older version of Pages goes way down. But at least it's something.
The only complete remedy is either keep up with "latest & greatest" OS version or use something other than Pages that isn't so quick to "break" file compatibility on older hardware no longer getting OS version updates. The hack workaround is to use third party tools to force new OS upgrades on hardware that won't naturally upgrade. As long as macOS is new, this kind of thing should work. Hacks can introduce other issues but they generally show that deprecated hardware can still easily run new macOS versions... Apple just chooses to leave hardware behind even when it is capable of handling any of the new stuff they introduce.
