"The download link for 2021 is gone from Autodesk site." Solution: Autodesk removes old versions after ~3 years. You must use the "Autodesk Virtual Agent" (chatbot) and ask for "Download AutoCAD Mechanical 2021." The bot provides a direct link.
If you have a .edu email address, Autodesk gives you full access to piece of software, including Mechanical 2021, for three years. Go to the Autodesk Education Community. No cracks. No viruses. Just a clean installer. autodesk autocad mechanical 2021 free download upd