Previously, frost walker used a BlockState#isAir check for the block above the water being air, allowing normal air, cave air, and void air. However, frost walker now uses a block state predicate that only allows the specific normal air block. This change can be seen in the below file:

https://github.com/misode/mcmeta/blob/60ebf87a2dc3683af90cf56a11a1599ca26712d9/data/minecraft/enchantment/frost_walker.json#L49-L57

As a result of this issue, frost walker no longer works at the top of the world (as outside of the world is considered void air), as well as positions covered by void air or cave air. Since these types of air function as air, I would expect them to all work the same regarding frost walker capability.