Steps to reproduce:
1) Summon a skeleton and begin taking damage from the skeleton
2) Run /fill ~ ~10 ~ ~ ~10 ~ minecraft:anvil
3) Look at death message: "Player was squashed by a falling anvil" 

How to see that the death message exists in the first place:
1) Summon a skeleton and begin taking damage from the skeleton
2) Run /damage 20 @s falling_anvil
3) Look at death message: "Player was squashed by a falling anvil whilst fighting Skeleton"

Note: Same issue for dripstone and falling block
"Player was skewered by a falling stalactite whilst fighting Skeleton"