Update – now working fine.
If you remove the Nuget package
Microsoft.CodeDom.Providers.DotNetCompilerPlatform
you can run Web Forms and MVC applications on GoDaddy’s shared hosting as Roslyn is not used and csc.exe is not uploaded to the /bin/Roslyn folder (and the Roslyn folder is not even created, obviously).
You would have expected their support teams to know about this.
Many, many thanks to Gary for those links, which solved the problem, and to everyone else who’s taken an interest and provided helpful comments!