The current downloads on VS Marketplace for VS 2017 and 2019 both have invalid signatures.
Hi @mattH - welcome to the community!
Thanks for pointing that out. The Symantec certificate we used to sign the releases expired in September which is why the current downloads are now displaying that warning (and the VS2015 version is also affected).
We have an updated certificate and we’ll publish new versions of the extension signed with the new certificate shortly (see SLVS issue #2017).
Update: the VSIXes on the Visual Studio Marketplace have been re-signed with the new certificate. Thanks again for reporting the issue.