Making coins un-spendable requires a restriction on consensus, requiring only a succesful soft fork.
Discussion
If the previous owners of coins lost their access or don't migrate it's their problem. We can not simply make 30% of the supply unspendable without reissuance and reissuance would be ridiculous. If coins in legacy addresses fall victim to a quantum hack - at one point - so be it.
The impact on Bitcoin's price by simply burning such a large amount of the entire supply would be insane as well.
It's all insane. At this point Q-Day is a faith-based doomsday cult. The implications of a QR signature scheme means that every non-institutional player in bitcoin is going to be long marched onto a layer 2 reservation, relegated to NPC status.