Given what you’ve written, the most significant challenge to the assets they once owned would come from a state Medicaid program — IF they used Medicaid to finance their “retirement center” move. States can “look back” from 5-7 years, depending on the state, to ask for repayment of benefits.
Other than that, and since the estate has been closed and is not being challenged, either by beneficiaries or the IRS, there is no particular reason to keep this documentation. But you really asked the wrong person. I am a saver of all documentation regarding taxes and finances! So if you have room in your basement, let it linger a while longer!