This bug is quite old, but it doesn't seem to have been reported yet.
When dying by taking void damage, totems of undying don't activate.
To reproduce
Hold a totem of undying in your hand
Fall into the void (e.g. in the end)
→ ❌ Note that you die while holding the totem, and the totem doesn't activate
Linked issues
is duplicated by 2
Comments 5
Again, @unknown, the wiki is not a valid source for intentional behaviour. While it is a useful source of information, it is no definitive proof that something is intended or not.
Also what exactly is the totem supposed to do? Heal you when you take fatal damage only for you to continue to take void damage and die a second time? Some people have keepInventory set to true and probably appreciate the totem not wasting itself like that. Or is the totem supposed to act like eating a chorus fruit does and teleport you to the nearest ledge? It's not like it does that when you fall in lava.
New statement after some discussion:
This is working as intended. Void damage is not normal damage - it can kill players even in creative mode, and there isn't meant to be any way around it.
Any indication that a totem should provide some effect to save you from the actual void damage (like teleportation or levitation) would be a feature request.
If the wiki is correct then this works as intended:
The totem can be used only once; it disappears after use. It does not save the player from death caused by void damage or the /kill command.