Apple issued negative bonds in some European country recently, if I understand this[0] correctly. Anyway, I don't think there's anything preventing a company from issuing negative bonds. Whether or not anyone would buy them is another question.

[0] https://markets.businessinsider.com/bonds/apple_incsf-notes_...

Except they don’t already in many cases.

https://www.barrons.com/articles/yields-go-negative-on-corpo...

Well one has to consider that it's pretty much the same big pool of money chasing both. This money pool thinks that German 10Y yield should be around -0.60%, while the Italian 10Y yields a juicy 1.62%. That says quite a bit about the future solvency issues around Italy.

Bottom line is that if there are still positive yielding bonds in a market dominated by negatives (we're not yet there) the risk of default is priced in.

If you can look into the future w that certainty you should trade stuff.