Check the first Unicode codepoints, 10 is defined there:
000A is LINE FEED (LF) = new line (NL), end of line (EOL)
It was already 10 in ASCII too (and the first 128 codepoints of Unicode are mostly the same as ASCII [I think there are a few tiny differences in some control characters]).
So to answer your question: it's neither 9 nor 11 because '\n' stands for "new line" and not for "character tabulation" nor for "line tabulation" (which 9 and 11 respectively stands for).
> Clearly at some point someone must've manually programmed a `'n' => 10` mapping
Check the first Unicode codepoints, 10 is defined there:
000A is LINE FEED (LF) = new line (NL), end of line (EOL)
It was already 10 in ASCII too (and the first 128 codepoints of Unicode are mostly the same as ASCII [I think there are a few tiny differences in some control characters]).
So to answer your question: it's neither 9 nor 11 because '\n' stands for "new line" and not for "character tabulation" nor for "line tabulation" (which 9 and 11 respectively stands for).
> Clearly at some point someone must've manually programmed a `'n' => 10` mapping
I don't disagree with that.