Fifth edition seems to have removed any means of healing undead.
The positive/negative energy duality of 3rd edition seems to be gone. Negative energy spells now do necrotic damage; some undead are resistant or immune to necrotic damage, but it isn’t listed as healing them. Classic negative energy spells like inflict wounds and harm don’t list undead healing as a possible effect.
For zombies and skeletons it might not matter, but liches and vampires would never risk their necks if they didn’t have some way to recover hit points. What is it?