Literature Stack Exchange has several questions about autorial-intent. An example from 2021 asked Do literature interpretations actually reflect what the author had in mind? There are also many other questions asking about the meaning of a specific literary work, implying that there would be only one (or one that would trump all other interpretations). I regard What is the inner meaning of “A Common Confusion”? as an example of this type of question.
My own literature classes at school also seemed to be based on the assumption that there is only one correct interpretation and that the correctness of this interpretation is determined by the author's intent. This was never made explicit but it was implied by the perennial question "What does the author mean?" and by the fact that only one interpretation was offered in response to this question.
However, as far as I know, scholars of literature had rejected authorial intentionalism several decades before I started secondary school (see Wimsatt and Beardsley's "intentional fallacy" and Roland Barthes's "the death of the author"). This raises the question why teachers still based discussions of interpretation on the notion of authorial intent. (I think they still do, thirty years later, but my focus is not on current practice.) Was this something that textbooks and theories on teaching literature encouraged? Or were such textbooks silent about this topic?
I am mainly interested in textbooks for teachers in Western Europa. (Teaching practices in Europe and the USA may have responded differently to new trends in literary theory. On the other hand, textbooks in non-democratic regimes may have had political reasons for teaching only one interpretation, namely the interpretation that was ideologically allowed.) The textbooks should focus on teaching literature in secondary education, nor primary school.
(Note: handbooks on how to teach literature should not be confused with handbooks that teach literature. The latter type may explain what a sonnet is, how to analyse it and maybe what the main periods in the history of English literature are. The former type is for teachers who teach this type of topics.)