Issue #6290 went 404

### Discussed in https://github.com/microsoft/vscode-cpptools/discussions/8253

<div type='discussions-op-text'>

<sup>Originally posted by **mitsukuri** October 11, 2021</sup>
There was an issue **#6290** which tracked the status of c++20 module support. Trying to check that today, I'm getting error 404.

What happened to the issue and how do we track the progress on that?</div>