Feather falling boots above (I believe) level 4 do not give extra protection. Feather falling 100 will still kill you in a 50 block drop. All other enchantments continue to increase their protection even above non-vanilla values.
MC-35394
Feather Falling Enchantment Not Incrementing Protection above IV
Resolved
Duplicate
0
Dustin Held
(Unassigned)
Unconfirmed
Minecraft 13w41b
Retrieved
Bugs with Effects and Enchantments > IV won't be fixed.