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In the old days, tent manufacturers would recommend that you unzip zippers before packing tents.

I don't see that recommendation anymore.

Are zippers different now? Did the recommendation ever make sense?

I've always just stuffed my tents into the stuff sacks without regard to zippers zipped or unzipped. Never had any issues. In my experience, sand and grit are what kill modern zippers, not storage or stuffing.

I know some older tents weren't as breathable, thus you always wanted to pack them un-zipped lest you wanted to trap moisture.

Now days, most tents seem to be made of more breathable materials.

Years ago, I heard that zipped coil zippers, when sharply bent, were more likely to deform than unzipped coils. I see nothing about this in modern literature.

On my 3-season tent, the part that I like to keep zipped is the mosquito netting/breathable fabric on the inside of the tent (that is, below the fly). That way, when I set up the inner tent, it is tensioned properly. Recently I noticed that the zipper came off the track (several times) at what appeared to be a fold line. The tent gets used at least once a month, so it never stays folded exactly the same way for long. The zipper may be coming off the track simply because that is the area of highest tension.

When I was a kid we had a big canvas tent with some kind of waterproof coating. You had to leave the door slightly unzipped when folding the tent, because otherwise the air inside the tent couldn't escape.

The zipper in question is on the totally breathable, netting and non-waterproof part of the tent. I always air the tent out the day I bring it home. So the water condensation is not what I worry about.

I last heard the claim that there was a mechanical issue (with folding zipped zippers) about 12 years ago, on an outdoor show.

I actually prefer to keep it zipper, because that's how it's supposed to be when you set it up, and having it zipped before packing means one less thing to do if, say, I have to set up a tent quickly in a drenching rain. Probably doesn't matter a huge deal, though.

Fold a closed zipper over and stomp on the crease, and you can probably force the teeth apart, i.e. create an opening away from the slider, which can be a royal pain to restore to normal working configuration. Or the teeth might break. Usually not an issue with tents, since you're rolling the fabric or doing gentle folds, not sharp creases.

It's also possible that design of the base of the teeth would make a difference in how sharply you can fold the fabric. Newer zippers might be more fold-resistant.

having it zipped before packing means one less thing to do if, say, I have to set up a tent quickly in a drenching rain.

Ditto that. I keep the fly attached with the door closed, so the inside stays dry while I lay out and stake the tent. I can even raise the tent without removing the fly, though it's not easy.

Faster to stuff with the zippers open unless you have a lot of mesh.
I'd also keep them open on internal pole'd single wall tent so you can peg the bottom and jump in quickly with the poles to finish setting it up.