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.
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The list by default of barrier, bedrock, end_portal, end_portal_frame, end_gateway, command_block, repeating_command_block, chain_command_block, structure_block, jigsaw and moving_piston are all completely immune to all damage by the wither. Both the initial explosion and all blue skull damage.
If, as you say, the tag doesn't provide immunity to the wither then I would say it is unfortunately named.
Does the wither destroy bedrock and other blocks in its initial explosion? The tag only prevents the blue skulls from overriding the immunity of some blocks; weak blocks like dirt or snow will still break to the explosion regardless of whether they're in the tag, which is intended.