The SQ version 8.9.x (LTS) seams not to be compatible with C#10.
The bundled version of sonar-dotnet is older than the first plugin version supporting C#10: 8.31.
How to get SQ8 to work with C#10 while it does not support (anymore) manual update of the plugin version?
To be honest, we’re still catching up on C#9 as far as I know, although SonarQube 9.2 did start to add some parsing support for C#10 structures. If you need that support, you’ll want to upgrade to the latest SonarQube version: 9.3 and keep up with subsequent releases. That way, you’ll get the new language support as it becomes available.