Thanks for this document and registering these profiles.  They appear to cover the necessary substance, and I have only two small editorial comments:

— Section 2.2.1 —

   A surrogate which occurs in text encoded in any transformation format
   other than UTF-16 has no meaning and may cause malfunction in
   software that encounters it.  In particular, it is impossible to
   represent a surrogate in well-formed UTF-8.

I think this is a bit confusingly put — I find the combination of “impossible” and “well-formed” to be correct but unclear.  The issue is dealt with later, in Section 3, but here might it make more sense to change “impossible” to “forbidden”?

— Section 3 —

   [RFC9413] emphasizes that when encountering problematic input,
   software should consider the field as a whole, not individual code
   points or bytes.

9413 says a bunch of things, and I can’t figure out what, specifically, you’re referring to here.  Can you add at least a section reference, and perhaps a quotation (as opposed to paraphrasing or summarizing)?