I created a Mega Mall -- a community lot with multiple CC rabbitholes. I have placed several vehicles on the lot (without parking spaces, if it matters for this question). OW is set to clean up abandoned vehicles. Will it nix the ones I placed or ignore them? I can't find a setting to tell it to ignore a lot during the sweep.

If Overwatch is set to clean up cars, it will probably delete those ones too. I tried to make a car shop and place cars on the savvy sellers rugs, but at 3am every morning, off they went!
They do stay if the lot is a residential lot, so I had to make a sim buy the lot and turn it into a home-run business and the cars stayed in place until someone brought them.
What do you want to happen with your cars, by the way?

I want the cars to stay on the lot. They are part of a fake showroom in the mall. I am building brick-and-mortar storefronts for each of the rabbitholes, and adding some other stuff that one might find in a mega mall for aesthetics. I am also considering adding a parking lot to the mall and smattering a few cars in there.

Sidenote: the only vehicles on the lot is the Extravagator 5000s, since it was the only one I could buy on a community lot (with buydebug active) while building it. And as I said, none of them are on parking spaces; dunno if that will make a difference or not.

Oh I see. How about using deco cars instead? I've seen some in the game, I can't remember where they came from, to be honest but they're there lol.

Havent seen anything like that in-game.

In any case, OW ran its cleanup and the cars stuck around. However, my sim was on the lot all night, so that may have affected it; will have to test it with the sims elsewhere and see how it goes.

I have a car park in the downtown area of Riverview. I placed 2 regular cars on the lot to give it the appearance of being used and have it set as Visitors Allowed. I'm happy to say that Overwatch has never removed those vehicles. They're sitting on regular parking spaces.

When I created the car park, I temporarily used Buzzler's Build Buy Restriction Choker mod so I could have a nice selection of vehicles. When I finished the lot, I removed the mod. The cars have stuck.

http://www.modthesims.info/d/450059

Not sure why it would have been necessary to remove Buzzler's Choker mod. I've had it place for so long that sometimes I forget that the game even places restrictions on commercial vs. residential lots. It's a very handy mod to have in place.

It wasn't necessary to remove Buzzler's mod, I think I removed it because part of it wasn't working right for me as far as I could tell in the game I was playing. I couldn't get stoves to work on community lots. Maybe I was feeling a little hissy or needed chocolate. Anyway, his mod worked perfectly for the cars I placed and they stayed put (which is what I wanted). I agree, it is a handy mod and I should try it again for stoves in a different game.

I've had a lot of trouble getting stoves to work on community lots, I think we all have. It might depend on the lot type. But that's not Buzzler's mod doing that, it's the game. The mod really only allows for placement, not so much usage. Had you been able to get them to work after being placed with the mod removed where they couldn't with it in place?