https://unicode.org/emoji/proposals.html has a list of factors for inclusion (example: “is legible and visibly distinctive”) and a list of factors for exclusion (example: “is overly specific”)

I don’t think Unicode.org has a nice list of rejected proposals, but examples are easily googled, for example https://charlottebuff.com/unicode/misc/rejected-emoji-propos...

Unicode's list of emoji proposal status: https://unicode.org/emoji/emoji-proposals-status.html