Sometime in the 20th Century, teachers, administrators, materials developers, and publishers got the idea that English instruction could be divided into separate language skills, abilities, or competencies. Listening + Speaking were classified as "Oral Language Skills." Reading + Writing became "Written Language Skills." Grammar (sentence structure with phrasing patterns) encompassed all of these. Vocabulary was derived from targeted subject matter and purpose. The purposes of this separation were to define, aim attention at, and utilize language-skills methodologies.

The issue of segregated vs. integrated skills instruction + its resolution is summarized in articles like "The Importance of Integrating Skills in the Teaching of English as a Foreign Language."

Of course, the concept of language-skills focus never meant that instructors could teach or learners could acquire any one of the four abilities without including others in lessons, activities, or even individual study or homework. Known as "Segregated Skills Instruction," these approaches often relied—and still depend—on the use of "strategies" or "action plans."

For more, you can read the full article here: https://worklifeenglish.com/blogs/news/integrate-language-skills