Is ‘outcast’ plural in “All of the outcast”?

If “outcasts” is the plural of “outcast”, how can it be used with a plural meaning in constructs like “the outcast” or “those outcast”?

Britannica, Wiktionary, Merriam-Webster, Collins, and a few other dictionaries all said the plural of "outcast" is "outcasts".

But I have seen "outcast" used plural, as in "the outcast" -- for example:

All of the outcast were waiting for a leader to return them to their former glory.

"Food and shelter for all those outcast?" he asked, doubtfully.

Is "outcast" also plural, or is that usage of "outcast" now obsolete?

An attributive adjective used in a definite Noun Phrase of the form The ADJECTIVE NOUN-PL (where that adjective modifies a plural noun) can always be used by itself to produce a subject governing a plural verb by omitting its noun.

Usually that unwritten noun works out to something like people or things or sometimes ones, but whatever it is this noun is readily recoverable in context.

Imagine a sentence like The poor are always desperate or The outcast have no course. These are adjectives, not nouns, and these subjects have no noun in them at all once the obvious bit has been left out. So it works out to The ADJECTIVE people where you skip people because it’s obvious in context.

The Cambridge Grammar of the English Language has invented the term fused-modifier head for these constructions, but other analyses use their own terms to describe these: "adjectival noun", "adjective used as a noun", "elliptical noun phrases", "headless noun phrases", "nominalized adjectives", "semi-nominalized adjectives", "substantivized adjective", and "zero-headed constructions" can all be readily found.

The difference between fused modifier-heads which remain adjectives (like the poor are, the outcast are) and fully nominalized adjectives produced by zero derivation (like the greens are, the outcasts are) lies in the degree of lexicalization and the syntactic category assignment of the word. In English, the plural suffix ‑s is a morphological marker restricted to the category of nouns. Because poor is an adjective, it cannot host this inflectional morpheme; to do so would require a categorical shift which the language does not permit for this specific subset of adjectives.

Now look at words like greens and outcasts which have undergone full conversion, also known as zero derivation. Clearly here these former adjectives have now been reclassified in the mental lexicon as nouns, for once a word has been categorized as a noun, it gains the ability to participate in the full range of nominal inflectional processes, including pluralization and possessives.

The difference is essentially one of syntactic flexibility versus categorical rigidity. When the poor functions as a noun phrase, it does so by “borrowing” the head position rather than becoming a noun itself. This is why it requires a plural verb (for example, ✅  the poor are) and yet cannot take a plural noun ending like ❌ the poors are.

Because adjectives in fused-modifier head constructions remain adjectives, these can be modified by adverbs, as in the newly poor are, the very poor are. However, once a former adjective fully converts and becomes an inflectable noun, that property is lost, making these ungrammatical: ❌ the recently outcasts. Instead they must now take adjectives as modifiers just like any other noun: ✅ the recent outcasts.

The plural of An/The outcast is is (Some/The) Outcasts are. Notice that that is a noun because it inflected into the plural. But that is NOT what is happening with your example sentence. You just have a fused-modifier here where the recoverable noun is itself plural. Your adjective is just an adjective, and it cannot have number in English.