The short answer is: they might be part of the N1 test (probably), but they’re so absurdly uncommon that most of the time, the only time you’d ever come across them is while you’re studying for the N1 test. So it’s not really worth the effort.
(And before anyone jumps up and goes “actually, I saw 嗣 in a novel just last week”, yeah, that’s an author’s prerogative. Just like they can use a word like “ichor” when writing an English book if they like.)