AI::TASK_STEALTH_KILL

void TASK_STEALTH_KILL(Ped killer, Ped target, Hash actionType, float p3,
  Any p4) // 0xAA5DC05579D60BD9 0x0D64C2FA

Stealth kill action name hashes:

stealth kills can be found here: Grand Theft Auto V\common.rpf\data\action\stealth_kills.meta
...
{
"ACT_stealth_kill_a",
"ACT_stealth_kill_weapon",
"ACT_stealth_kill_b",
"ACT_stealth_kill_c",
"ACT_stealth_kill_d",
"ACT_stealth_kill_a_gardener"
}

Only known script using this native: fbi4_prep2
EXAMPLE:
ai::task_stealth_kill(iParam1, Local_252, gameplay::get_hash_key("AR_stealth_kill_a"), 1f, 0); ai::task_stealth_kill(iParam1, Local_252, gameplay::get_hash_key("AR_stealth_kill_knife"), 1f, 0);

Also it may be important to note, that each time this task is called, it's followed by AI::CLEAR_PED_TASKS on the target