Adult chickens do not die of lava on a slab.
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In 1.16.0.59, the lava damage and fire ignition point of lava was reduced to be 0.6 meters above the bottom of the lava (see a similar report: MCPE-45633). With this change, the bottom of the lava damage and fire ignition point was reduced to about 0.4 meters above the bottom of the lava, making about a 0.2 meter gap for entities to be damaged, leaving a majority of the block space free from harm.
Edit: This is extended to vertical lava streams as well. Shooting an arrow through a lava stream will not ignite it unless it collides with the 0.2 meter area.

Relates to MCPE-46074.
Same with cows