This feature allows Wurst to bypass servers that kick Fabric users. Since Fabric doesn't tell the server what mods it is using, some servers may opt to prevent Fabric clients from connecting.
Note: The mixin must be done from ClientConnection.class, otherwise not all the packets will be captured.
Can you give some examples of servers that do this?
Most server owners won't block fabric, but they will install a plugin that alerts them when fabric users connect.
Here's a plugin I found that blocks fabric: https://www.spigotmc.org/resources/lib-hates-mods.78202/
This has been merged into the
v7.7-mc1.16.3 branch and will be in master when v7.7 is released.