Open studio, press play, in the explorer window open "Chat", copy "ChatModules", stop testing, and finally paste the ChatModules folder back into chat. This will now allow you to modify part of Roblox's default chat functionality, including removing that message.
To remove the message, open the ChatModules folder, and open the script "ChatCommandsTeller." Now, scroll to the bottom of the script. You should see this:
1 | if ChatSettings.GeneralChannelName then |
2 | local allChannel = ChatService:GetChannel(ChatSettings.GeneralChannelName) |
4 | allChannel.WelcomeMessage = ChatLocalization:FormatMessageToSend( "GameChat_ChatCommandsTeller_AllChannelWelcomeMessage" , "Chat '/?' or '/help' for a list of chat commands." ) |
Delete it, and the message will be removed.