How do I create a custom editor for VS2015?

The extensibility project templates were refactored into item templates in an effort to make the templates more usable. For example, you can add multiple items like commands, toolwindows etc to a single project.

That documentation is definitely outdated. I'd recommend using the Editor_With_Toolbox sample, or perhaps the WPFDesigner_XML sample (from the VS Extensibility Samples on GitHub), as a starting point for your project.

According to: https://msdn.microsoft.com/en-us/library/dn916149.aspx?f=255&MSPPError=-2147217396

It seems like the ability to create custom editors in any language but C++ was removed in VS2015.

I think the microsoft dokumentation, for these topics, is outdated.