that's called "OP_RETURN" which is a terminal UTXO (can't be spent)

witness data is another matter. this is a scar that bitcoin acquired in the blocksize wars. it was a mistake, they should have just used schnorr signatures, like we finally have with taproot, that complicates it and obscures the fact it's just schnorr signatures. (tweaking is purely application level matter, the chain only sees a schnorr pubkey). we can deprecate it in favor of taproot. it takes up less space so the benefit is cheaper transactions.

removing the 80 byte mempool filter on OP_RETURN won't lead to as much spam as the witness data can (due to the 4x lower price), but removing the user's ability set a limit at all is malicious.

i would appreciate it if you would not lie to someone who genuinely wants to know, because either you do, and are intending to confuse my friend, or you don't, which means you are an ignorant disinformation replicator.

either way, go fuck yourself, goat fucker.