While you cannot buy a new permanent license for AutoCAD 2018, you can still download it if you have an existing entitlement:

Students and educators can access older versions by verifying their status on the Autodesk Education page . Permanent Installation & Licensing

: The specific product key for AutoCAD 2018 is 001J1 .

No – perpetual licenses do not include upgrades. You would need to buy a new subscription.