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On the latest discussions, there are proposals to increase the minimum mediawiki requirements to MySQL/MariaDB 5.5 and allow binary or utf8mb4 (not utf8, 3 byte utf8), https://phabricator.wikimedia.org/T161232.
Utf8mb4
should be enough for most uses (utf8 will not allow for emojis, for example), although I am not up to date with the latest unicode standard changes and MySQL features supporting them.
I dont know about mysql, but in unicode emojis are like any other astral character, and utf-8 can encode them in 4 bytes*.
* there are some technicalities in that some emoiji are actually multiple code points that join together to render one glyph (for example country flags take 2 codepoints to make a flag for a total of 8 bytes, there are some modifier characters that simply change the skin colour of emoiji, etc) however this should not affect anything on the db layer as they are just treated as multiple characters.
-- Brian