most of the CFD brokers send emails these days, disabling OIL CFDs or keeping Close-only orders... with notice that all positions will be closed if Spot price goes below $2-3... btw. not sure if any MT4/MT5 platform can go negative:-) I doubt they are built for that
The real price of Oil which is the one found on the Spot market is the one to pay attention to. According to my broker, yes price can go negative but we cannot place orders there because of the platform. A whole lot of Bull.
The reason that we saw negative pricing in the futures market yesterday was because of a specific problem with settlement mechanism of the May 1st WTI contract (which requires—by law— delivery into the landlocked tank farms at Cushing, Oklahoma) rather than wider national or global build in storage (contrast with the seaborne Brent price for May).
Cushing Oklahoma is a literal choke point in the supply network of oil in the south-central U.S. All pipelines of finished product lead to Cushing in the inner U.S.
It seems to me that anything can happen on the market, but such situations are not typical, so they can be called exceptions to the rules and in order to avoid them you need to qualitatively assess the risks and be careful during the events related to fundamental analysis.