Are there places which are legally required to only accept test result certificates?
Not in NL. Why does this matter?
Any QR code which can be printed, can be copied. There's a vested interest for the operators of these systems to reduce the copyability of these codes (think: vaccine passports). Embedding a temporal code, changing the QR code every few minutes, and invalidating old codes, essentially OTP, is the most straightforward and industry standard way of doing this; it's inevitable.
We are right now in the process of…
"Legally" part looks like a stretch goal to me, however.But precedents that simply overturn other laws -- or as applies to the matter at hand, in the United States at least: the 4th Amendment - no.
Anyone can make "forward looking statements". If they're going to be useful and not dismissed as hysterical fear-mongering, then it helps if they're actually credible.
In some societies, "ability and availability to install mobile applications" is now taken for granted. This comes from the fact that in some societies the vast majority of the members take sheepish "what others do must be right" behaviour as granted. Those mentalities are enablers for regulations that may claim to mirror alleged "de facto" situations - making relevant prospects credible.
Surely many state supported sacrifices to privacy of the recent times (in some regions) were "supported" with "Don't they have FB anyway?" (thing is, of course, no they don't - a few would never).
> projecting and extrapolating from recent historical events
When some of the «recent historical events» come in "support" of already established desiderata of some parts (say, cashless society - to be intended as option dropping), creating an acceleration towards said trends, then they get relevant.