OP_RETURN_199 ​

OP_RETURN_199 fails the script immediately, but it must be executed (for example it does not render a script invalid if it is in a branch of an if/else statement that doesn't get executed). It is a synonym for OP_RETURN.

OP_RETURN_199 is part of a group of opcodes that are not defined directly in bitcoin core, (the OP_RETURN_199 name comes from rust-bitcoin). Those opcodes are OP_RETURN_187 to OP_RETURN_254, and all render a script invalid if executed. This means that they essentially behave like OP_RETURN, but their use would make a script non-standard, and therefore a transaction that would include one of them would not be relayed by nodes by default.