It is restricted by the standard Bitcoin client implementation. Neither the transaction scripting language[0] nor the transaction[1] restricts it, so you would need to do it yourself or write a custom client to embed a message.
[0] https://en.bitcoin.it/wiki/Script
[1] https://en.bitcoin.it/wiki/Transaction#Output
It is restricted by the standard Bitcoin client implementation. Neither the transaction scripting language[0] nor the transaction[1] restricts it, so you would need to do it yourself or write a custom client to embed a message.
[0] https://en.bitcoin.it/wiki/Script
[1] https://en.bitcoin.it/wiki/Transaction#Output