They introduced some kind of caps (don't remember the details) on negative pricing quite early on, from what I understand it would have been very lucrative in the last decade or two to get into grid-scale battery storage without those caps.

One thing I remember is Flensburg building, pretty much on a whim, a water storage tank with immersion heater, an investment that amortised within a month or two as they were literally getting paid to fuel their district heating.

There's got to be some rules as to what you can do with electricity you by at negative prices, e.g. not just put an immersion heater in the ocean, maybe some prioritisation as to who gets the energy first just as there is on the production side (fossils have to shut down and pay if they don't do that fast enough while renewables get to produce energy), but overall I don't see why there should be a limit on negative prices.