Why is that? I tried to add a thumbs-up to a post recently because we don't have an emoticon for it, and it just automatically deleted it from the post. Is Xenforo seriously not compatible with UTF-16? I can post emoji on PHPBB boards just fine.
Xenforo is not UTF-8 compatible by default and strips all code points in the emoji range. Mainly because most MYSQL installations didn't support true UTF-8 when it was being designed. It's not a difficult fix, and I'm not entirely sure why they didn't use the same method as VBulletin (convert all invalid codepoints to html entities), but it's not one I'm willing to make atm.
https://xenforo.com/community/threads/true-utf-8-support-mysql-utf8mb4.90122/
It's UTF8, just not UTF8MB4 (4 byte codepoints), which is what's required for emoji. It's MySQL that doesn't support it, if you read through the link I gave.
