I don't know about "most", but it is probably safe to say "many." Sallie Mae retains ownership of 25% of all outstanding student loan debt. If the debt was forgiven, I think it is safe to presume their equity stock price would go to 0. Look here for the largest holders of that stock:

https://finance.yahoo.com/q/mh?s=SLM+Major+Holders

Much of the other 75% is I'm sure also distributed into many other common mutual funds that are used in people's 401k plans.