I'm saying this because I've gone from Silver 1 to Silver 3 this weekend but...

You don't always get demoted after one loss at 0 Lp, so there kinda is.

I lost like 6 games at 0 LP once and didnt get demoted. And no i wasnt in division 5, I was plat 2

You generally have to go one losing streak to drop down. I'm not sure what the criteria are though. It may me MMR based or you might have to lose a couple once you hit 0 LP. It's probably MMR based. So if your MMR drops too far below that of your division when you're at 0LP and you lose, you get dropped.

you can think as anytime you have 0 LP and haven't been promoted recently as a match that could make you drop in rank. You might lose that match and drop or you might need to lose another one. It really doesn't matter whether or not there are "demotional" matches or not since all they will accomplish is make you be even more frustrated and more likely to jump at teammates for doing bad or stressing yourself out for doing badly since you now know for certain that if you lose the match you will get demoted. It's the total opposite for promotional matches since for those you are hyped that you might be able to advance in rank.

Would be pointless since not everyone is demoted after the same amount of losses.

If you have promo helper, you need one win after 100 lp to be promoted To be demoted, you need one loss after reaching 0 lp

same thing