As asked here: #112440 (comment)
When I ran the commands from this guide:
cd src/libraries/System.IO.Compression/src/
dotnet build /p:ApiCompatValidateAssemblies=false
dotnet msbuild /t:GenerateReferenceAssemblySource
A ZLibException was unexpectedly added to the ref file, even though it's explicitly excluded in the CompatibilitySuppressions.xml file of System.IO.Compression.
I do not know if the GenAPI Task is currently configured to know how to locate any CompatibilitySuppressions.xml file and determine which APIs to skip. But since the addition to the ref was unexpected, we should look into this.