I no longer think the unsupported models list relates to the ability to open and edit documents. I think it relates to the ability to update to the latest version of the application.
Why did I previously think the unsupported models list related to the ability to open and edit documents?
In late 2018 I discovered that I couldn't open documents that I edited on my MacBook (in Pages 7.3) on my Mac mini (in Pages 5.6.2).
So I came to the Support Community. I read that Macmini3,1 is on a list of unsupported hardware that cannot open Pages documents created on Pages 7.3 (here: https://discussions.apple.com/thread/250023398). And, since my Mac mini is Macmini3,1, I thought that explained why I suddenly couldn't open documents on my Mac mini that had been edited on my MacBook since I updated to Pages 7.3.
I decided to downgrade to Pages 7.1 on MacBook (having read that there was an incompatibility between Pages 7.2 and Pages on El Capitan, my Mac mini being on El Capitan - https://discussions.apple.com/thread/8568370). I also downgraded from Pages 4.3 on my iPhone & iPad to Pages 4.1. And once again all was well and I could edit the same document on all my devices.
So why do I now think the unsupported models list does not relate to the ability to open and edit documents?
First, I discovered that my Mac mini was on the list of unsupported models before the release of Pages 7.3.
I went through the Pages plists backed up in my Time Capsule. The first one was created on 13 October 2016 (when Pages 6.0 was current) and did not have any lists of unsupported models. The next one backed up was created on 16 November 2016 (when Pages 6.0.5 was current) had a list of models including Macmini3,1 and it has remained on all the plists since then. Yet I was able open and edit all my documents on all devices including the Mac mini in 2016 and all the way through to late 2018.
Second, the list of Mac models is under "OSXUpdateInfo" on the plists.
I don't particularly understand plists (I just use computers to create things) but it seems to me that the list of Mac models comes under "OSXUpdateInfo" which always lists the then current version Pages together with a minimum OS version. Accordingly I think this is the information about what is the current version of Pages, which OS is necessary to run that current version and which Mac models are unable to run that current version.
Third, our other Macs are on the unsupported models list but are still able to open Pages documents created in Pages 10.0.
We have a MacBookAir3,1 and an iMac12,2. They first appear on the list of unsupported models on a backed up plist created on 14 March 2020 (when Pages 8.2.1 was current) under "OSXUpdateInfo" with Mojave listed as minimum. I don't have a backed up plists for when Pages 8.2 was current and they may have first have been added to the list of unsupported models at that time.
In addition to them currently being able to open Pages 10.0 documents suggesting that the unsupported models list does not relate to opening documents, neither of these Macs can run Mojave or update to Pages 8.2.1 which suggests that the list relates to the ability to update.
In these circumstances, I don't think my Mac mini appearing on the unsupported models list is why I was having problems opening Pages documents. All the plists I have backed up (apart from the first one created on 13 October 2016) list "OSXSharingInfo" with Pages 5.6 as the minimum. I think the intention was to be able to share with all Macs running Pages 5.6 (and newer) and, therefore, I don't know why my Mac mini could not open Pages 7.3 documents it but I suspect it relates to the OS being El Capitan.
As for collaboration, information about this first appeared on a backed up plist created on 25 April 2017. This was when Pages 6.1.1 was current (I don't have any backed up plists during the period Pages 6.1 was current) and the minimum version for collaboration was the current minor version. It continued like this until Pages 7.0 when the minimum version became "minimum v2" but remained the current minor version and another minimum version was added which was Pages 6.3 (then the last minor version) presumably meaning it was no longer necessary to be on exactly the same minor version to collaborate. Pages 6.3 has continued to be the other minimum version presumably expanding the ability for users of different versions to collaborate. In Pages 7.3 "minimum v2" was no longer the current minor version but the instead the previous minor version. However this expansion did not last and Pages 8.0 once again had the "minimum v2" as the current version. And so it has continued with "minimum v2" being the current minor version.
