“His father beating his mother was the usual in the past”. Is not ‘usual’ an adjective here?

Yeah! It is. It is called an adjectival noun. It simply refers to an adjective that functions as a noun.

Adjectives are often used as nouns:

Adjectives Function As Nouns
Ellipses of Noun Phrases: Ellipsis is the shortening of a phrase. In this case, adjectives can act as nouns and can be a subject, object, or complement in a sentence.

1/In the examples illustrated bellow, the adjectives acting as nouns form a part of the subject.

Examples:

  • The hardworking are not snob
  • The poor are underestimated.

‘the hardworking’ and ‘the poor’ are adjectives acting as nouns. 
The full noun phrases could be: 

  • The hardworking (people) are not snob
  • The poor (students) are underestimated.

2/Furthermore, they can function as objects:

Examples:

  • She is a fan of the dummy.
  • He has an eye for the aristocrats.

‘the dummy’ and ‘the aristocrats’ are adjectives acting as nouns. The complete sentences could be:

  • She is a fan of the dummy(minds).
  • He has an eye for the aristocrats (ladies).

REMEMBER:

The definite article ’the’ is combined with these adjectives so that they can be used as nouns. These adjectives are always plural and take a plural verb.