I don't want this to sound like I'm defending having a proprietary debugger, but a lot of the tools that people use are proprietary even while they think they're using FOSS. For example, Visual Studio Code + Python? All that IntelliSense is a proprietary Microsoft binary and can't be used with non-Microsoft products. Are you running an open-source fork of VSCode? You can't run that plugin. Want VSCode + JavaScript + IntelliCode? Proprietary again.

As the link notes, there is Samsung's open source debugger, but it's not the one that is used by any of the common IDEs (VSCode, Visual Studio, or Rider).

It's certainly not an amazing situation that I want to be defending, but most people look past the fact that they're running plenty of proprietary stuff inside their VSCode editor and if you're looking past proprietary extensions for JS or Python, it makes sense to also look past proprietary extensions for C#.

You can download the official .NET debugger but its license only permits you to use it with VS and VSCode.

Luckily, there are various third-party debuggers available. E.g. from Samsung (MIT licensed) and Jetbrains (proprietary for their IDEs).

https://github.com/Samsung/netcoredbg

The debugger is largely proprietary.