Autodesk offers AutoCAD 2025 for free to students and educators for one year (renewable). This gives you full access legally.
Legally, software does not become "abandoned" just because it is old. However, if you have a paid invoice for AutoCAD 2006 from 2005, you are legally allowed to re-download the installer from certain archival sites (like Internet Archive) as long as you use your own legal serial number. Autocad 2006 with key full version
If you need AutoCAD 2006 functionality but cannot afford a modern license or cannot find your old key, consider these legal alternatives: Autodesk offers AutoCAD 2025 for free to students