If you absolutely need the 2021 64-bit version:

. This version supported both SSL and IPsec VPN tunnels, which were essential for the surge in remote work at that time. Amazon Web Services Key Features of FortiClient 6.4 (2021) Free VPN-Only Version

: A lightweight version for Windows 64-bit that provides basic connectivity to FortiGate devices without central management.

Select and browse the 6.4 directory to find the specific build you need.

Unlike modern versions, the 2021 client does not support SAML (Single Sign-On) via embedded browser well. You may need to use "Legacy" authentication mode.