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)
MC-166389
Silverfish suffocate inside of soul sand
Resolved
Fixed
4
muzikbike
(Unassigned)
Confirmed
Very Important
Hitboxes
mojang_internal_2
1.15 Pre-Release 2
1.15 Pre-release 3
Retrieved
I thought this was a feature used for silverfish proofing your house