Upgrading to ASP.NET Core 3.0 went pretty smoothly overall, and first indications show overall page load time improved by nearly 20%. Given there were no changes to YetaWF, I have to assume that's all thanks to 3.0, maybe endpoints?
But then it came time to publish the first site. And it didn't work. Investigation showed, the EXE on Windows 10 built for deployment with IIS simply did not load. It pretended to run, but it didn't.
I noticed I had an <OutputType> tag in my Website project file:
Once that was removed, dotnet publish worked like a charm and created a proper EXE.
Go figure. It seems like this MSBUILD option breaks dotnet publish.