It doesn't depend on the application unless the application shares the same pointers between x86 and arm which doesn't make any sense to me.

Otherwise they're right, it's not the intersection that matters but just the total bits available

Eh? The values you can store in the tags are absolutely dependent on the number of available bits. That’s a simple type safety problem. This requirement is architecture independent.

You can’t cram 8 bits of tag in 7 bits if the latter is all the architecture has available. Hence why you have to design for the smallest reliable target.