Add support for utf8mb4 encoding to make possible use (direct storing) of the unicode characters outside of the Basic multilingual plane range (such as Emoji).

https://dev.mysql.com/doc/refman/5.7/en/charset-unicode-conversion.html

Just for the info, some BB utf8mb4 use stats (which I could get):

Invision Community - utf8mb4
XenForo - utf8mb4
Flarum - utf8mb4
Vanilla Forums - utf8mb4
vBulletin - utf8 (default), utf8mb4 (supported)

myBB - utf8 (default), utf8mb4 (supported)
Simple Machines - utf8
PunBB - utf8
phpBB - utf8