I am now trading on a demo account for several months and I am currently looking for a broker. I just realized that not many brokers offer a negative account balance protection or even some guarantee of the slippage.
For example in 2015 EUR/CHF, how much slippage was there? I can’t seem to find any hard numbers. I also assume this is going to be wildly different per broker but I would still like to see some numbers.
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It’s market and broker dependant. If your concerned about slippage - use a garenteed stop.
People lost their houses and Brokers went bust !
We have at least ONE member who was short at the time ! 
In Europe, the new proposals will ensure that you can’t lose more than you have in your account. I expect the reputable brokers will extend that.
But to avoid slippage on individual bets you will need to use guaranteed stops which some will allow at a cost.
Negative balance is no longer going to be a thing. If you can’t wait until July, when the proposal is going to be applied, then just ask your potential broker.