Regenerating a planeswalker will prevent it from being destroyed as a result of a “destroy” effect (like the one from Reaper King). However, it won’t prevent that planeswalker from being put into its owner’s graveyard when its loyalty becomes 0.

Reknit can regenerate an artifact, creature, enchantment, land, or planeswalker. If you regenerate a noncreature permanent, the next time that permanent would be destroyed that turn, instead tap it. If that permanent had any damage on it (because it had been a creature earlier in the turn), that damage is removed.