I’m no economist but it seems to me once they pass a certain threshold of “volume” on “negative interest rates” a Depression is pretty much inevitable.
I don’t see why negative interest rates would cause a depression. Negative inflation (deflation) is known to cause depressions but not negative interest rates.
Negative interest rates simply means that you’re willing to pay a government to hold their debt rather than hold currency. Maybe there are storage/security costs associated with the currency, or risks inherent with individual banks that don’t exist or are less with government debt instruments.
It may make a statement about investment opportunities. For example, say the media is talking up a recession and you don’t want to invest in case they are successful.