I was going to ask a question about the origin of the notation L1, L2, etc. for designating the order of learning languages. This seems to be a historical question.
Do we want this type of question? Why or why not?
Asking about the history of language learning seems fine to me, in general.
I considered asking about the history of SRS in language learning, but Wikipedia answered my question, so I didn't bother.
Asking about the history of the terminology L1 and L2 seems fine to me. It of course needs to be done in a way that otherwise fits within site guidelines, but that shouldn't be hard to do.
While questions about the history of languages would be off-topic (we have Linguistics Stack Exchange for that; see history, comparative linguistics and historical linguistics), questions about the history of language teaching or language learning are on-topic.
Both language learning and language teaching methods have evolved throughout history, even though the scientific study of second language acquisition is quite recent (roughly since World War II).
I consider questions about history on-topic because there may be things we can learn from history (if only to avoid mistakes made in the past).
This site is about language learning, not about linguistics. We shouldn't ask questions like "Have teaching methods for language learning changed over the last 50 years?" or "Which languages have had the most impact on English".
On the other hand, history should be acceptable if it is relevant to language learning. For example, how learning an ancestor language would make a descendent language easier to learn. It certainly belongs here if it can help people learn languages, instead of just being "trivia".