Since the Bitcoin Stamps outputs are already unspendable, it makes perfect sense to mark and drop them from the UTXO set.
There is no consensus change involved in not storing a provably unspendable output, it's just an implementation detail with no interoperability implications and doesn't need a BIP. Bitcoin core has long done so for several types of unspendable outputs, e.g. outputs over 10kb and ones starting with OP_RETURN.