Missing HEIC support (and Apple support in general) is not an issue of quality; it's an issue of "knowing your customers". I doubt there could be any certification body for that.
The test portal "not responding" when receiving an unexpected file format is a quality issue. Not making it clear to the user that the upload has failed before it is too late is another quality issue.
Plot twist: There is. It's a standard clause in software procurement contracts, especially for government.
"The application must be supported and tested on browsers with at least 10% market share, as defined by the yearly Gartner report"
Replace browser with device or what is relevant for the project.
I'm also wondering how many iPhone users have that .heic setting at its default value, as opposed to having switched to .jpg.
> Not everything needs to be tested on mobile
It does when it requires taking and uploading a photo in the year 2020. Especially when its target users are high school kids. What adult is going to use anything other than their phone to take an off-handed photo to upload somewhere these days? Much less a child who probably almost never uses a traditional computer.