History of OP RETURN
The OP_RETURN opcode has a controversial history that has culminated to it being used as the primary mechanism to store arbitrary data on-chain. The original functionality was intended to behave as a return operation that ends execution of the script. The original implementation had a critical bug where one could steal anyone's Bitcoins and was quickly disabled by Satoshi Nakamoto.
OP_RETURN was originally intended to return the top value on the stack. Given the script:
True would be returned from the script.