A strong indication that many questions about notation are on-topic is that the tag (notation) has (as of today) 84 questions, at least half of which are well received.
I personally think the "Why was the notation [...] introduced as opposed to the notation which was formerly in use"-type or "Who introduced [this essential notation] and how did people express [the corresponding thought/...] before?"-type questions (e.g., How did mathematicians notate the empty set before $\varnothing$?) are interesting and potentially of the most significant historical context. Equally appropriate are, in my opinion, those questions which are about notation and working practice of scientists; e.g., when it comes to composing texts, like Writing Mathematical Symbols in 20th century.
As opposed to the questions of the (poor) form "When was [...] introduced/first used?" which are most likely answered by "According to Jeff Miller's website, [Name] was the first to use it in [Article, Year]."; history should be more than the sole dates, at least whenever possible.
Here is a related quote from the introduction (p. 1) of F. Cajori's book A History of Mathematical Notations:
In this history it has been an aim to give not only the first appearance of a symbol and its origin (whenever possible), but also to indicate the competition encountered and the spread of the symbol among writers in different countries. It is the latter part of our program which has given bulk to this history.
The rise of certain symbols, their day of popularity, and their eventual decline constitute in many cases an interesting story. Our endeavor has been to do justice to obsolete and obsolescent notations, as well as to those which have survived and enjoy the favor of mathematicians of the present moment.
In conclusion, I think for a question about notation to be good, it has to have some sort of historical context and for it to be on-topic, it should at least be expectable that it gets an answer involving an interesting historical context.