https://wccftech.com/rdr2-zombies-mod-video-lets-you-taste-the-potential-of-undead-nightmare-2/

An RDR2 Zombies mod is in the works. As it stands, it may well be the closest you'll get to an Undead Nightmare 2, given how Rockstar developers recently said there are no plans for anything like that.

Tarek Hamad (Online Producer on Red Dead Online) - It’s not something that we’re focused on right now. We’ve obviously just released Frontier Pursuits, so our focus is building on top of that and expanding the player’s experience in the world at the moment. But you know, we never say never, so we’ll have to see where the future goes.