I want to make mod, that disconnects player from server when certain message appears in chat (I've done listening with mixin) and after all checks i need to disconnect player with message something like bad message was found blah blah and after and then I can return to multiplayer menu (all this must be clientside)

This is not a fabric bug report.
Use discord or the discussions tab to ask for help.

@warjort Imo such questions are fine for issues.

@SashaSemenishchev You can look into how the /kick command is implemented.

gh discussions should be used for questions imo

@warjort Imo such questions are fine for issues.

You should make a template for it then, instead of pointing people to the discord/discussions when you "create new issue".

@SashaSemenishchev You can look into how the /kick command is implemented.

Kick is server side. You need to look at net.minecraft.client.gui.screen.GameMenuScreen.

It's basically:

MinecraftClient client = MinecraftClient.getInstance();
client.world.disconnect(); // tell the server you are disconnecting
client.disconnect(screenToShowWhenDisconnecting);
client.openScreen(screenToShowAfterwards);

Never done it myself. But I bet it only works properly if you are on the render thread?

If you still need support, please ask in one of the dev channels of our official discord server.