Previous version support change describes this change for not only AutoCAD but all Autodesk products. .
“Starting August 31, 2019, Autodesk will start enforcing its technical support lifecycle policy for all current versions of Autodesk software and previous versions (typically three versions back), listed on the subscription and maintenance plan eligibility lists.”
March 2021, product activation aligns with the Lifecycle policy so if you by then have AutoCAD 2018 and older you might find that you can use it as you no longer can activate it.
Autodesk have a dedicated forum now.