Can my PhD admission offer be rescinded due to COVID-19?
I don't think any of us here really knows. My best bet is that most universities will choose to honor offers of graduate fellowships and certainly will honor acceptances that come without any financial support.

But it's a legal question that will likely depend on the laws of the country and state the university is in, whether the university is public or private, and a number of other possibilities. It may also depend on the financial position the university is, or will be, in. You're unlikely going to get a definitive answer here, but you can always ask the people at the university that gave you the offer for guidance.