More context from Gargron here
https://mastodon.social/@Gargron/110130305097514687
he clarifies this is not a confirmed feature and promises no one will sue for customizing mastodon source code
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wants to disable custom emoji in usernames because people fleeing Twitter are confused - and wants people to know that modifying mastodon to keep that feature is Against The Rules

That's definitely what people who have worked hard to build this community want - unilateral decisions from an out of touch tech bro