I've seen a few people include this in their lot descriptions: "No sims have ever lived on this lot prior to uploading!"

Why does that matter? Does it make a difference to anyone? Is that something I should strive for when building a house that I plan to upload? I often move in a sim while I'm building to help me test the routing, turn lights on and off, push in the chairs, etc. It's easier to adjust these things while the build is in progress. What is the reasoning behind not doing that?

Lived-in lots (especially if you've used an ingame mod to keep all the furniture) can have issues, like used items littered around, stuck items, sim references, the occasional bugged object (if it's been interacted with), and other fun little annoyances.

Packing lots with sims still living there will also pack a copy of the sims, cut-off relationships, and whatever other baggage they've got. Not recommended to put in a neighborhood you want to keep clean. It's not a "BFBVFS" worthy level of bugging up your game, but can potentially cause some issues down the line.

(April did a lot of testing on NH corruption, and found that lived-in lots where sims had been packed alongside, were less of an issue than the general simming population have previously thought, though the concerns still have merits and it's possible to severely overcrowd the hood, so better safe than sorry. Not sure if she mentions empty but previously occupied lots anywhere, as that video is long and it's been a while since I saw the full video - it's worth watching, though. https://www.youtube.com/watch?v=2cB67DGDXFQ )

It's possible to clean lots for your own use (I'd highly recommend it if you want to re-use previoulsy occupied lots in your own game).

1: Extracting lots and reusing them (copy lots to the bin, minus sims, CC references kept but not packaged)
https://thesims2.livejournal.com/6049797.html
https://2fingerswhiskey.livejournal.com/160934.html
(either one is fine)

2: Cleaning them - manually or with CH's tool (CH's cleaning globe is linked at the bottom of Joandsarah's post)
https://modthesims.info/showthread.php?t=609188

It's also an upload requirement, not optional - those statements are there for the moderators as much as the downloaders.

https://modthesims.info/rubrics/4168/ - also links to the #2 option above as the method of cleaning, and suggests play-testing on copies.

What I always do is to build the lot, then make a backup of the hood.
Then I can move some sims in and test the lot, and figure out what needs improving.
I go back to the backup copy to make these changes, then make another backup before testing that.
This way, you will always have an unplayed copy of the lot to upload.

There are a number of other ways you could do this, but essentially it's just keeping an unplayed version at all times.

Personally, I've recently used a hood without any sims (clean/empty templates, and all that jazz) to build new lots in.

I also have one for cleaning previously occupied lots, as I had some I really needed to reuse (a "throwaway" hood, putting them in the intended hood or the lot bin after processing).

I first extract/copy the lot by moving it to the lot bin via the method described above (this doesn't transfer sims, only the lot, so the lot can technically still be occupied), then clean it up outside the game (described in Joandsarah's post) - which means the lot is already pre-cleaned before I start using it. Then I check it out ingame, deleting/replacing potential problem objects (newspapers, plates, items sims can have "relationships" with, etc.). CH's cleaning object can be used here, too.

I haven't uploaded a lot of lots, and the ones I have uploaded (elsewhere) have been unoccupied versions. I don't think I'd be comfortable uploading previously unoccupied ones, but I don't mind using the cleaned versions in my own game. Any potential issues arising are just going to be my own problem, not someone else's.

I have always built my lots in an empty neighborhood that never gets played. It’s just safer. If they need testing, I package them up and install them in a different, testing only neighborhood.

Thanks! That's exactly what I'm doing with a lot I'm working on now!

I am the one who encountered the issue with sims leftover files inside packed lots. I wanted to share a community lot with fireplaces lit by sims who have lived there, but I found an error message after attaching a file so I just got rejected halfway in MTS upload wizard. I guess they invented some checking system that prevents lots of dangers and can be trusted.