We are realizing that the UDv2 guideline for PROPN is a pretty radical departure from the previous approach, which for English followed PTB guidelines.

Note that PROPN is only used for the subclass of nouns that are used as names and that often exhibit special syntactic properties (such as occurring without an article in the singular in English). When other phrases or sentences are used as names, the component words retain their original tags. For example, in Cat on a Hot Tin Roof, Cat is NOUN, on is ADP, a is DET, etc.

It makes sense to give function words their usual tags even within names.
But enforcing this policy would require reviewing all multiword names in (at least the English) treebanks to convert PROPN's into NOUN, ADJ, etc., depending on context.

Moreover, it's not entirely clear what the principle is behind the distinction. In the sentence "I'm reading Cat" (short for Cat on a Hot Tin Roof) should it be NOUN or PROPN?

A fine point is that it is not uncommon to regard words that are etymologically adjectives or participles as proper nouns when they appear as part of a multiword name that overall functions like a proper noun, for example in the Yellow Pages, United Airlines or Thrall Manufacturing Company. This is certainly the practice for the English Penn Treebank tag set.

Is this saying that an adjective in some names can be treated as PROPN but in other constructions (such as Cat on a Hot Tin Roof) it is obligatorily ADJ?

(Related: #664)