I'm not sure where the documentation for previous versions of the Lab Services Rest APIs (i.e., pre-August Update 2022 for Lab Services) reside. However, there are samples at the older GitHub location for Lab Services at https://github.com/Azure/azure-devtestlab/tree/master/samples/ClassroomLabs. It includes an SDK sample.
The GitHub repo also has a preview version of the Lab Services PowerShell module. (Warning: I recommend only importing for current session as this version might cause confusion if you are using the lab plan-based PowerShell module also.) If you pass in a '--debug' to a PowerShell cmdlet you should be able to see the actual rest calls being made.
Was there something in particular you were trying to accomplish? I'm familiar with both versions of the api and might be able to help you.