Send messages to offline players with this Data Pack! It's related to / inspired by the Essentials (plugin) /mail command. Just type /trigger MessageBook in the chat, then your message on the first page, sign the book by filling in the player's name and it'll send the message! The latest received message will always display at the bottom of your chat. You can save the book for later use, as well!
The Data Pack (Minecraft Version 1.16)
Very bad and outdated (1.14) YouTube video: