It is stands against wither_boss initial expolsion, explosions of skulls boss throws, but fails when boss aggred to player and chasing him, breaking Dark Blocks, Obsidian and other weak blocks.
This happened on singleplayer, and also on server.

MC 1.12.2
Forge 14.23.5.2847
Unique Crops 1.12.2-0.1.99
(and nothing else!)

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That was the last hope by progressing VolcanoBlock modpack, I don't know nothing else to stop wither boss, it's only way to craft NetherStar via Alchemistry recipe... for now.
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BTW, code of this block contains
EntityEnderman.setCarriable(this, true);
Indestructible block, seriously? Trollo-lol ;-)
Just in case, that block is really indestructible, for wither boss too.