An OP_RETURN is a provably prune-able output, these outputs do not need to be stored in the UTXO set but will be permanently part of the blockchain. These outputs can be used to add arbitrary data to the blockchain, creating a timestamp that it existed at a given time.
OP_RETURN Bot will create an OP_RETURN output for you, you just need to pay the fees.
Note: this is a fork that will only do 0.1 sat/vB transactions, fill or kill style. There is no guarantee that your transaction will be mined.