All of our understanding of the Bible is mediated through things outside the Bible.
At the very least we have our understanding of the Biblical languages. We rely on lexicons (dictionaries) which in turn rely on lots of extra-Biblical texts.
We use the commentaries and explanations of the Bible from past ages to guide us, whether or not we end up agreeing with them.
We use archaeology and history to clarify details, resolve ambiguities, and fill in the gaps of the text. The Biblical texts generally assume quite a lot of background knowledge from their readers, so people like us who live in very different cultures will need a cultural boost to understand the full implications of the Biblical texts.
So history, archaeology, and other external evidence is all quite on-topic for this site. All answers are expected to provide supporting evidence for the interpretations they give, and some of that supporting evidence will be external to the Bible. Some times it will be the crucial supporting evidence.
Whether questions should be asked about that external evidence... it might depend on the question. We still want the focus to be on the text. The evidence should bring us back to the text to help us understand it more. The text shouldn't be used as a springboard to external matters ("this random verse mentions sheep - what breeds of sheep did Israel keep?").
As to the specific phrasing you raised in the question, "what the external evidence says about a text", I'd say questions shouldn't be asking that. Recognise that all of our understanding is mediated through external knowledge and such an overt statement is not needed in a question. It also makes it seem like "external evidence" is some objective interpreter of the text, which is not the case. We, the people, interpret the text, using all sorts of knowledge, so it is we that are saying things about the text. Instead, just ask about the detail in the text you want to know more about, and if answers don't provide supporting evidence for their interpretations then you can flag them.