Idea

Detect server's signing requirement without requiring the first message to be sent

Reasoning

Would make it faster to know the status and therefore make decisions based on it; simplify the indicator

Other Information

A competitor mod used this mixin to achieve this (1.20.4):

    @Inject(method = "onServerMetadata(Lnet/minecraft/network/packet/s2c/play/ServerMetadataS2CPacket;)V", at = @At("TAIL"))
    private void onServerMetaData(ServerMetadataS2CPacket packet, CallbackInfo info) {
        if (packet.isSecureChatEnforced()) {
            // implementation
        }
    }

I don't know why, but so far it appears to be spoof-proof, meaning it will not false alert on servers that hide the toast on vanilla clients, such as Hypixel and other viaversion ones.

Related: #437

https://discord.com/channels/757941072449241128/992979156767883375/1190552197101002884

Or [the server] can say signing is required and not refuse to process unsigned messages, likewise converting them to system

So this would still not be feasible.