If you are running ancient VMs (Windows NT, OS/2, or early Linux distros), the virtual hardware in 5.5 is often more stable for these guests. The Better Alternative: VMUG Advantage

: ESXi 5.5 reached its end of general support on September 19, 2018 .

If you choose to use a key found on GitHub, connect that host to the public internet. Use it strictly in an isolated, air-gapped lab environment. Publicly leaked keys are often flagged by security software, and using them in a corporate environment can trigger licensing audits and significant legal penalties.