Keylight is available in Adobe After Effects. Unfortunately, it is not available in Adobe Premiere Pro directly.
If there's a Clip in your Timeline that you'd like to key with Keylight instead of Ultra Key (included with Premiere Pro), right-click the Clip Boundary and choose Replace with After Effects Composition.
Once in After Effects, right-click the Layer and choose Effects > Keying > Keylight.
After Effects 2023 - Right-click Layer and choose Effect, then Keying, and then Keylight
I usually save the resulting After Effects Project next to the Premiere Pro project file.
The Keylight user guide is included in the Keylight folder that is in the Plugins folder. This the file path under macOS:
/Applications/Adobe After Effects 2023/Plug-ins/Effects/Keylight/docs/Keylight_1.2_AE.pdf