Is this usage of "all" considered archaic?
Solution 1:
I wouldn't class it as archaic; it's uncommon, to be sure, but still sometimes used. Applying purple to your prose is not always a bad thing, especially if you're trying to set a mood for a scene; the key is to use such descriptions in moderation and to blend them smoothly into the surrounding narrative.
Unless you're writing deliberate homage to '30s pulps, in which case your prose should go between purple and ultraviole(n)t.
Solution 2:
I don't think there's anything at all archaic, or even uncommon, in OP's use of the word "all". Here, for example, are thousands of written instances of it was all sunshine (and a few hundred for it was all doom and gloom, in the interests of balance). An ever more common usage which shows no sign of falling into disuse is he was all smiles.
Nor is there anything unusual about the fact that OP doesn't explicitly repeat the implicit [it was] in the second clause - this is perfectly normal in both speech and writing.
At the level of sentence structure, there's nothing at all remarkable about OP's example. About all one can say is that "chill air" and "dazzling sunlight" are both very slightly "flowery" and "poetic" - but it's obviously OP's intention to write an evocative descriptive passage, so that's fine.
Solution 3:
It's not a particularly common phrasing when used as a descriptive, which might give it the air of archaism, and you particular example is somewhat poetic, adding to that nuance. It sounds like a poetic elision of 'the afternoon was all...' where 'all' is an adverb, modifying the predicate.
But the adverbial form of 'all' is not at all archaic or formal. Looking at its very long OED entry, many uses are very common nowadays. One particular phenomenon is very informal, namely its use to introduce reported speech:
She was all "What's up?" and I was all "nuh unh" and then he was like facepalm, so I bugged out.
'all' acts like a synonym of the ubiquitous 'like' (which is a synonym of the more formal 'said').