Based on 1.11 decompiled using MCP 9.35 rc1
The server sends the motd as plain text component (method net.minecraft.server.MinecraftServer.run()). This requires the player to use the legacy formatting character (§) because the string is displayed as plain text for the client.
The motd should be a text component which is then parsed and send to the client.
This would require old motd messages to be converted to a text component.