I think it is occasionally used with "the," i.e. "the conscious" (referring to the conscious part of your body, for example). Adjectives sometimes become nouns this way, like "the poor"
I searched the Corpus of Contemporary American English ( https://www.english-corpora.org/coca/ ) for 'conscious_n', which means the token "conscious" with a 'noun' part-of-speech tag.
There are five results. All five of them are tagging errors:
If we scan to get enough info, then model the cells well enough, and have enough computers to run the simulation of the models, then the input-output of the emulation of the brain will be the same as the input-output of the original brain. It will act like it is conscious. [adjective, modifying it]
Well, first we work on working the body together, so that we can go places with both of us conscious. [adjective, modifying both of us]
Lady Bertram looks barely conscious. [adjective, modifying Lady Bertram]
In a few years, he believed, this institution would be needed in Ukraine, as new conscripts became more religiously conscious. [adjective, modifying new conscripts]
It is in this sense that Rahner means that grace is conscious. [adjective, modifying grace]
Examples 3 and 4 are so far from being nouns that they're being modified by adverbs.
It seems safe to conclude that in fact there is no nounal use of the word "conscious".
> Adjectives sometimes become nouns this way, like "the poor"
That isn't actually what's happening in "the poor". The position occupied by the token poor in that phrase can be filled by all kinds of things:
God loves everyone equally. The rich and the poor, the just and the unjust, the sane and the schizophrenic, the possessed-of-billions-of-dollars and the penniless...
Do you want to argue that "possessed of billions of dollars" is a noun?
We can apply our in-passing observation from earlier and contrast the fully-awake with the barely-conscious. Here, as above, it's impossible for conscious to be a noun, because it is being modified by an adverb. And it's... dubious... for barely conscious to be a noun phrase, because it is headed by conscious, which we know isn't a noun.
> Is my impression correct, that in general "the {thing}" is a noun phrase without implying anything about {thing} itself?
Yes, with some minor caveats:
1. Some people prefer to see "the {thing}" as a 'determiner phrase', where 'determiner' is the name for the part of speech to which the belongs. You can call it a 'noun phrase' without losing anything meaningful. 'Noun phrase' is definitely a better term if you're not deep in the technical weeds of grammatical analysis.
2. There are conclusions you could draw about {thing}, but they're more complex than "it's a noun". It's fair to just not talk about them.
3. In language, there are always problems somewhere for any analysis. (Which is why an unbroken chain of transmission can have Latin on one side and French on the other.) I wouldn't even say that a noun phrase with that structure exists at all in an example like "The more you say it, the more I think it". But that particular construction is weird enough that I'm perfectly comfortable saying it's just outside the scope of your qualifier "in general".
