" they received a notice from the IRS indicating that estate tax returns may be due going back to 2018."

I would take that as IRS also saying that estate income tax returns may NOT be due going back to 2018.  And that the IRS computer is programmed to suggest to every EIN applicant that tax returns may or may not be due.  The notice of the new EIN always tells people they have to file for the current year, when in most cases they don't. 

I wouldn't respond to a generic IRS computer notice.  Was all the income reported by the surviving spouse?  Did someone think to step up the basis for depreciation?  What basis and depreciation was used for the 2024 sale?