In the past, structure voids were unbreakable, similar to structure blocks and command blocks.
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Wiki states that Structure Blocks were changed from a blast resistance of about 18000003 to none or 0.
Erik Broes
The blocks should have no use at all. If you cannot break them they would be 'walkthrough barrier' which they are not intended to be.
[Mod] Neko
Erik Broes
Set confirmation status to community consensus, as the wiki also states this change.