According to Apple's documentation GLKit is in beta for Mac Catalyst.
However, when attempting to run our iPad app with Mac selected as the device, we get:
GLKit is deprecated and is not available when building for Mac Catalyst. Consider migrating to MetalKit instead. You may need to restrict the platforms for which this framework should be linked in the target editor.
Upgrading to MetalKit is not an option for us, unfortunately.
Is GLKit available for Mac Catalyst? If so, are there any special settings we need to set? If not, I'm presuming this is an error in Apple's documentation.
GLKit is a deprecated framework, regardless of platform. The range of operating systems for deployment are iOS 5.0–12.0 / macOS 10.8–10.14 / tvOS 9.0–12.0. Individual pages like this list that support range. I wouldn't trust that main page sidebar mentioning Catalyst.
Since Catalyst only exists in the macOS 10.15 / iOS 13 (or later) world, it's Metal or nothing. There are tutorials and videos to help with transitioning.