A very good answer to the OP’s question is transhumanist Max More’s online paper: “The Terminus of the Self”.
We know that people can be revived after their breathing and heartbeat has been stopped for several minutes. Therefore, the critical indicator of death would seem to be reduced to brain activity. But how often is that actually measured to determine if someone is dead? Only for comatose persons to determine if the plug should be pulled. Right?
Brain activity would appear to be the last sign that someone still has minimal function to be considered alive. If that goes, then that’s it.
Nobody who was frozen has ever been revived. There have been rare cases when people (usually children) with severely lowered temperature and frozen limbs have been “defrosted”.