I've updated the Graph.Community project to include SiteDesignRun information for a site.
await gsc.GraphServiceClient .SharePointAPI(mockWebUrl) .SiteDesignRuns .Request() .GetAsync();
This capability is included in v3.9.1, which was published to NuGet on Aug 18, 2020. Version 3.9.1 aligns with the Microsoft Graph SDK v3.9, and targets .Net Standard 2.0.
There are several items on the Roadmap if you wish to get involved.