The paid license key for ESXi 5.5 provides the following features and benefits:

: In some cases, if you have a valid perpetual license for a newer version (like vSphere 6.x), the licensing portal may allow you to "downgrade" that key to a 5.x version for legacy hardware compatibility.

Leo checked the vSphere Client—the old fat client, not the web one, which had been clunky even a decade ago. A warning icon sat next to the host summary: Evaluation Mode. 60 days remaining.