There is the statute of limitations, and then there is the time for creditors to present claims if a probate is started, or if the trustee of a living trust uses "notice to creditors" procedures available in some states.  Both of those timelines raise the issue of when the debt became uncollectible.  Looks like it would have been 2018 at the earliest, so the tax statute has not expired.  But that doesn't mean 2018 income should be reported on a 2020 return, just because of some piece of paper.