To clarify, I was only talking about the short term rates, those should have an arbitrage with e.g. central bank deposits. Long duration will usually have higher interests (I think 10y bonds from confederation wasn’t super negative, like -0.2% at most?)
I’m not aware of any such law. Many people seem to believe they can’t go negative on nominal terms but I don’t think it constitutes a guarantee. The dollar can fall vs the CHF too and nominal returns on individual treasuries held to maturity can be negative in CHF terms, as can CHF hedged treasuries and as have TIPS in the recent past.