Some of the dead languages in Unicode arguably were never "real" languages and history disputes if they were actual written forms for a language or merely local doodles. Their relevance to Unicode is apparently there is so much academic work still trying to sort out said history that papers are written all the time with those dead symbols. (For all we know, some of them may even be ancient emoji. :P )
The difference between "formal" and "informal" is a fuzzy zone. It's almost like we're making up all of language as we go along. Unicode seems like a fine place to standardize our growing need for emoji. Also, for what it is worth, all of WingDings is also in Unicode and has been since early on.
My only criticism with emoji in Unicode is that I do think it means that Unicode should reconsider its stance on previously closed proposals for constructed languages like Klingon and Tengwar and maybe dedicate a section somewhere outside of the basic multilingual plane (BMP) for them.