Update: The texture will appear on whatever block you placed where the shulker box was, and if you start mining the block while walking towards it, it'll continue the damage. So you can break a shulker box, place obsidian, walk and mine and it'll break it in a second.
Confirmed for 20w15a
Update: The texture will appear on whatever block you placed where the shulker box was, and if you start mining the block while walking towards it, it'll continue the damage. So you can break a shulker box, place obsidian, walk and mine and it'll break it in a second.