Since you do not have natural regeneration in Ultra Hard Core mode players would need to play much safer and would get slower times, leaving more room for optimization and an overall harder challenge to players. I understand that setting an Ultra Hard Core world would be hard, so maybe someone in the community (that is reliable) could make an engine or mod pack. This would also decrease the chance of cheating or modifying the game.

https://www.speedrun.com/mc_hhh

There is a half hearted hardcore which you only have half a heart which is harder than you suggested but slightly similar