I've noticed a few NuGet packages reference an underlying .Core package from their solution.

Microsoft themselves have been using this convention in recent packages Microsoft.AspNetCore.Mvc.Core, Microsoft.AspNetCore.SignalR.Core and Microsoft.EntityFrameworkCore.Sqlite.Core.

However, this answer on StackOverflow from years ago says this is a bad naming convention.

The difference is about Assemblies vs Namespaces.

The Microsoft.AspNetCore.Mvc.Core namepsace doesn't seem to exist. The classes are all simply in Microsoft.AspNetCore.Mvc. The Microsoft.AspNetCore.SignalR.Core.csproj on GitHub has the <RootNamespace> set to just Microsoft.AspNetCore.SignalR.

I've usually applied the same logic as namespaces to my assembly names, but I'm beginning to like the convention of having .Core assemblies and having the root assembly name (i.e. without .Core or any extension) being a metapackage that references all the sub-assemblies. Microsoft seem to be taking this approach now.

So I've renamed some projects in Firestorm. For example, Firestorm.Engine.Core and Firestorm.Endpoints.Core. These are to be exposed as NuGet packages too, but as with Mvc and SignalR, the packages would usually be referenced by the main package your application would reference.