Is life after death experience unethical to prove or disprove by doctors risky thing so doctors rarely attempt this with a drug ketamine?

Killing someone to investigate this process is not unethical - it is criminal.

No one, can PROVE there is life after death.
If someone returns to a living state after some event, like cardiac arrest, then they were not <<truly>> dead.

It is NOT "provable", so the question does not arise. By definition, someone who is DEAD has no further communication or interaction with anyone in THIS world. If by some chance there is some resuscitation of someone who was earlier pronounced dead, then that was obviously a mistake, and they cannot have been "dead" dead. Whatever their demented brains may cause them to think they remember for that period is pure fantasy. Dead is dead, and will forever remain so.