Found this thread while checking to see if Mojave supported Pages 4.3. Guess it doesn't. Oh well, it was good while it lasted.
There's one detail I haven't seen mentioned. Apple has said that Mojave is the last system to support 32 bit apps, BUT they also said that High Sierra was the last one to support them "without compromise"
So, clearly Pages is run with compromise on Mojave. High Sierra did work fine for me, though I can't guarantee it wouldn't break some specific functionality you use.
I got lucky: the new Pages actually now fulfills my needs. The last thing I needed was facing pages, and they added that about a year ago. So I'm now able to use the new Pages on mac, iphone and ipad. It's pretty nice.
But they have obviously not ported all features. On the bright side, they are slowly continuing to add features back in. Too slowly....5 years. But, they are doing it.
So High Sierra is the end of the line and you should be able to stay there for a while until you need to upgrade. But, I encourage you to make a list of Pages 4.3 features you need, and check if they've been added. I hadn't looked at the new Pages for a while and was surprised to find that everything I needed worked. And new features continue to be added.
I wouldn't hold out a ton of hope, but I also suspect that more than 0% of the people in this thread actually could use the new Pages, so it's worth looking into before switching completely. If you search my threads I had a few on making it work for my print and web workflow.
(Am currently using pages to scrivener to html. Both new and old Pages produces surprisingly good html output, which is why I continue to use it for my highly specific workflow)
Good luck all! I've got one thing to test on new Pages before I update to Mojave, to make sure everything works. And I plan to keep one old machine on Sierra or High Sierra so as to have access to Pages 4.3 just in case.