Minecraft wiki entry for Soul_Sand, associated text: Because soul sand is not as high as a full block, standing near lava while on soul sand sets the player on fire. Standing near water while on soul sand extinguishes the player, though direct contact to fire sources still hurts them.‌[Java Edition only]
Soul sand has inner collision boxes every eighth of a block. However, these collision boxes have an effect only if entities are clipped into soul sand.
If a water current is pushing the player diagonally against a wall and soul sand is the block adjacent to the player's head when the current ends, it causes the player to jump.
If snow or carpet is over soul sand, the effects of it being a partial block are avoided because the player walks on the cover block, not the soul sand.
Before Village & Pillage, soul sand used to generate in iconic four-block-deep layers between y levels 60 and 65. It also occasionally generated on the floor of caverns and coasts of lava oceans below, only if it generated in the same column at Y=60 or above. Toward the end of Village & Pillage development, the generation range of soul sand was reduced so that it exists only below y level 34, close to the lava sea level. As a result, soul sand is much harder to find in Nethers that were generated in the 1.14 and 1.15 versions.