When would any of that ever come up at even the most advanced level of ESL teaching?! It wouldn't. You are unable ti counter the real crux of the matter and are attempting to distract by confusing academic study of the history of English with teaching English as a second language.

It's interesting, yes, but a different field to frontline ESL teaching.

You don't need to boggle students with the history of spelling-sound causality in order to teach them how to pronounce each phoneme. It's nice to know from an academic standpoint, but really has nothing to do with frontline teaching, be that at A1 level or C2.

And lesson plans should be more than a rough structure/guideline, it helps to keep track of where your students are and where they are going, especially when preparing them for exams such as Cambridge or IELTS. Of course there may be deviations as things come up in the class, you have to go where the natural learning takes you, but then the lesson plan is helpful in planning the content and structure of the next class.