uh iOS is what powers iPhones. macOS (previously called OS X) is what powers Macs.
I am not sure what you expect to get from contacting Avid, there is the official compatibility table and release notes of what Pro Tools is officially supported on different operating systems. That does not necessarily mean other things will not work but that is what Avid has officially tested enough to say they work. And all Avid support would reasonably do is quote that same information...
https://avid.secure.force.com/pkb/ar...tibility-Chart
What issues with accessing what older files are you worried about — you mean old sound designer files? Generally “accessing older files” is not the usual concern with Pro Tools and macOS compatibility, it’s wether Pro Tools will run at all or without significant bugs/limitations.
Your license may let you run only up to Pro Tools 12.5 so be careful trying to upgrade Pro Tools.
What actual problem are you trying to solve?
Pro Tools 12.x is not only not officially supported on recent macOS releases but it just will not run OK. So if your goal is to get onto a recent macOS version you will need to purchase a subscription or a license renewal. Issues with Pro Tools 12 on different macOS releases have been discussed in many past threads on DUC you can search for (Google search works better than the forum search).
You also need to work out separately if all your third party plugins and interface drivers are compatible with whatever macOS you want to upgrade to.
As always it is safer/better to not just apply an in-situ macOS upgrade but to do a full clean install on a spare drive ir partition, install Pro Tools etc there and test it works. Eventually cloning that hack to the boot drive if it works OK. Do not use system migration assassin (assistant) to move apps or drivers off your old boot drive, install them all from scratch, you can use it to move user accounts and files.