A creature has had it’s hit points reduced to 0 by a Specter and has been revivified. Do they need to be able to benefit from a long rest in order to have their hit point maximum increase again?

An Echo Knight's echo is an object. It's a translucent, gray image that occupies space; it isn't a creature; and it can be targeted. Object.

It's like you're targeting a moving painting or a hologram with substance. #DnD https://t.co/kmCZdEnNmu

— Jeremy Crawford (@JeremyECrawford) March 19, 2020

If a specter reduces your hit point maximum to 0, you die. Here's the true horror: if someone revives you, your hit point maximum is still 0, so you're dead again. There are few ways around this—the wish spell being one.

Undead are loathed in D&D worlds for such horrors. #DnD https://t.co/c7YWXYGcGX

— Jeremy Crawford (@JeremyECrawford) March 17, 2020

The thing here is that this is a reduction of maximum hit points that doesn’t have an end time except for a long rest. Therefore, if something else gives you hit points, or heals you, it fails because your maximum hit points determines… your maximum hit points you can have. You're correct, @WilliamBosleyDM. 👍

— Jeremy Crawford (@JeremyECrawford) March 19, 2020