The expected behaviour would be for silverfish to not suffocate inside of soul sand. However, mobs should still suffocate inside of soul sand (as well as farmland and grass paths) if their head is within the solid 15/16 of the block. (This logic should be applied to other solid non-full blocks as well; see MC-134360)
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discovered while testing
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MrRedeminator
I thought this was a feature used for silverfish proofing your house
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Yeah, this could be mistaken for something that was intended