Prior to snapshot 21w20a, helmets used to cause extra durability damage when they fell on a player. This has been the case since release 1.4.6 when anvils were first changed to do more damage to helmets than other pieces of armor. This mechanic was likely broken due to MC-206839 being fixed since anvils and pointed dripstone have a similar damaging mechanic when falling.

How to reproduce:

1. On any version from 1.4.6 to 21w19a, drop an anvil from around ~10+ blocks up onto yourself. Notice the durability lost.
2. On any version from 21w20a or later, drop an anvil from the same height. The durability lost on the helmet will be much less than expected.