The debugger interface is open (it's part of the CLR itself), and so is the debug symbol file format. However the debugger interface is low-level, making writing a debugger engine from scratch fairly complex. The complaint is that Microsoft uses a restrictive license for their higher-level wrapper around the low-level API.

But it should be possible to write independent .NET debuggers -- I believe Jetbrains has done so, Rider can debug C# code without the problematic Microsoft component.

Roslyn (the open-source compiler) can create open-PDB debugging files that are also an open-source format. You are 6 years too late to make that comment.

And I think lldb works with .NET apps.

Even if Microsoft did release an OSS debugger with less features/performance than the proprietary debugger, you know the narrative would instantly change to complaining about keeping features/perf back for paid users.