The issue is that the list is infinite (anything not specified is UB), so actually removing any finite amount of UB from the list won't make it shorter.

(only slightly tongue-in-cheek, I do believe that removing silly things is worthwhile).

The list of UB categories and rules is not infinite. The list of UB programs is, as is the list of all non UB programs.

It is not obvious to me that the list of categories is not infinite (unless the final category is "everything else" of course)

To be undefined behaviour, it must at least be valid syntax. The syntax is described in a finite document. Also it only gets executed by a finite machine, that has a finite number of finite descriptive documents.

The list of unspecified behaviour is infinite, but the list of undefined behaviour is well defined and finite ;)