How familiar are you with the people who will be participating in your match?
In an open (to the public) shoot this unknown is probably the key reason why guns are abandoned rather than reholstered or slung.
Some matches are an exception and applicants are advised ahead of time that they will be expected to perform these actions. Attendees are more likely have some idea how the equipment works and they have also probably selected appropriate gear.

Some slings will leave the muzzle pointing at the feet of the person carrying it and some will hang the weapons where it flirts with the 180.
You can expect to see a variety of slings, not all of them good.

There are similar considerations on holsters: Holsters with butt forward cant will pose an issue with the 180 during reholster. Some holsters don't stay open well enough to receive the muzzle without using the off hand. Some don't retain the gun well enough for going prone. Having been an RO at open local matches, I've seen problems with these actions 'off the clock' enough to make me leery of asking some of those folks to try it with the timer running.
In some cases the holster is good but the belt is inadequate to support the holster or the shirt gets tangled on the pistol on its way into the holster.
(Know your clientele.) So...we use abandonment boxes/tables. Not that reholstering can't be safely accomplished on the clock, but the level of experience locally doesn't really instill enough confidence to make it a requirement in our stages.

I have written some stages where I had the shooter transition to the handgun after using the shotgun. We placed the abandonment box such that it would strongly encourage the shooter to engage some handgun targets strong hand only until they reach a point where the abandonment takes place.