What attacks actually exist that reduce someone's max HP, but do not specifically state what happens at max HP of 0?
As far as I am aware, all monster abilities that can reduce max HP to 0 state that the creature dies (and optionally may be brought back as some undead creature, depending on the exact creature/attack used to reduce the max HP to 0).
As an example of an attack that does state what happens, consider a Wraith's Life Drain attack (MM, pg. 302):
Life Drain. Melee Weapon Attack: +6 to hit, reach 5 ft., one creature. Hit: 21 (4d8 + 3) necrotic damage. The target must succeed on a DC 14 Constitution saving throw or its hit point maximum is reduced by an amount equal to the damage taken. This reduction lasts until the target finishes a long rest. The target dies if this effect reduces its hit point maximum to 0.
Are there any attacks or effect that reduce a creature's max HP to 0, but do not address whether or not the target dies?