The 1.14 snapshots (from 19w05a) added a new tag for blocks called "wither_immune" which prevents the Wither boss from being able to break them. By default the list includes bedrock and command blocks which act per previous versions, immune to all damage from the Wither.

Adding a block to this list should protect them from damage in the same manner, but any non-default block added to the list is only protected from Wither projectiles and not from the Wither's initial "birth" explosion as all the default blocks are.