How do I keep sims from going off the grid in the Sims 4?

@beeke404 Are you talking about NPC Sims who have been culled? The way you "favorite" them is open the Manage Household screen and the Other Households tab, then find the Sim or household you wish to favorite, and then click on the heart icon in the lower right corner of the household tile. This will move that household from the Other Households tab to the My Households tab. This should prevent them from being culled. I have never had a Sim culled from the My Households tab, though I have heard others say they have. You can also save a copy of that household to your Library if you want to be double safe. That way, if the game culls a Sim or a household you can import them back into your game from your Library.

The mod you are referring to is likely MCCC. It has a Sims Flags menu where you can select various "flags" including a Flag to Never Cull. I use this flag on my Sims and any NPC townie that I do not want the game to ever cull. You can remove the flags if you ever change your mind about that Sim. You'd have to disable aging somewhere otherwise they'll eventually die from old age. I disable aging in my game options and then I manually age up Sims in the Manage Households/ Edit in CAS screen.