The Meaning of Life, the Universe, and Everything.
You can use the tellraw command to give custom text in chat, you can also use command blocks to create custom names for items and entities. Use online command generators, as they make things much easier. You can use mcstacker to generate a tellraw command here: https://mcstacker.net/ or you can use any other tellraw command generator.
If you are talking about the actual command output that the command block puts into chat, I don't think that's possible.
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Remember those versions that minecraft pranked us with? Specifically:
Those are still downloadable! Watch this video for 2.0:
To download the other ones you need to make a folder in the versions folder for minecraft and put the client and JSON file for the versions in there. They all need to be named the same aside from file extensions. Once you do that, you will be able to choose that version when making a new profile with the minecraft launcher.