Do they? That's one of the things that's not clear at all. There's no indication of how an enemy finds you. Is it an action? Free? Based on passive or active checks? The most straightforward reading for me is they the enemy actually has to take an action: "which becomes the DC for a creature to find you with a Wisdom (Perception) check."I don’t think so. If a creature’s passive Perception is higher than your Stealth check, then presumably (and according to what it says in the new rule) it would immediately find you, unless, as @FitzTheRuke suggests, the intent is to remove passive Perception, but the new rule doesn’t say that.
So that ancient red dragon with a +16 perception now has to use their entire action (or legendary action, since it's an option) to find someone they don't know is there? Blech.