What grammatical evidence is there for allocating the word point to a particular part of speech in phrases like nought point five or nine point nine nine, where these phrases would represent the numbers 0.5 and 9.99 respectively (as opposed to being a description of how those numbers are 'spelled' or written). I assume that the word originates from the noun point representing the punctuation mark, but wonder what they are in their current usage in modern English - but more importantly what evidence there is for any particular categorisation?
I'm not sure the 'point' has a classificiation all by itself. It adopts the classification of the other words with which it shares an identity. In this case, 'nought point five' is a number and each of the words is therefore a noun.
It might be sufficient to say that point is one component of the noun phrase "nought point five". However, the individual words in noun phrases can typically be labelled with their parts of speech. (E.g. in the noun phrase "noun phrase", phrase is a noun that heads the noun phrase, and noun is a noun adjunct). However, "nought point five" doesn't have this type of structure.
I'd suggest that writing out the individual components of a decimal number is analogous to spelling out the letters of a given word. For example, given the word number, spelt out 'phonetically' as an-you-am-bee-ee-are, we might enquire about "bee". The usual meaning of the noun bee (a type of insect) isn't relevant.
Large dictionaries list each letter as a single-letter word. Each such word is defined as a noun, denoting the letter with which it is spelled. - Category talk:English one letter words, Wiktionary
By that analysis, point would be a 'noun' that denotes the corresponding portion of the number under discussion.
However, consider the following:
In the English language, words can be considered as the smallest elements that have distinctive meanings. Based on their use and functions, words are categorized into several types or parts of speech. - Part of Speech Overview
The key is that "words" are "the smallest elements" that have "distinctive meanings". I'm not convinced that "point" in the phrase "nought point five" has such a distinctive meaning. Stepping back a little, I'm not convinced that "point" is the "smallest element" in "nought point five". It might be more helpful to analyse that 'phrase' as an open compound word instead, like "living room", where each component doesn't have its own part of speech:
An open compound word is created in cases when the modifying adjective is used with its noun to create a new noun. This isn’t quite the same as a noun with a modifying adjective. - Compound Words, Grammarly
For example, living is an adjective, but in the open compound word "living room", it doesn't really function as an adjective - the whole compound word needs to be treated as a "smallest unit" when assigning parts of speech.
"Nought point five" should be considered a compound word, where "point" is one of its components that should not be separately assigned a part of speech.