If I have a question about the history of English like “When did [some thing] become incorrect?” or “Was [some thing] different during [some time]?”, would that be off-topic?
Yes, history of the language is very much on topic.
For example, mentions of Old English has ~6200 results, asked starting from the very inception of the site.
I don't think this is an exact duplicate but here's a meta question Are Old and Middle English on on topic? (full disclosure: the main answer there is mine).