"As long as they have users developing in GOPATH mode."
It's documentation written when GOPATH was still a widely used for development.
I think nowadays it's less of a concern to have a module first code base with GOPATH unsupported. It's nicer to use a V2 branch. Or delegate V1 to a V1 branch and maintain V2 in master.
I’m not disputing that things may have changed since the post was written. But the comment I responded to claimed that the linked post didn’t recommend copy-into-subdirectory, which it clearly does.
How is a branch nicer than a subdirectory? Off the top of my head, it seems like backporting bugfixes might be easier using a branch (if you're lucky, you can just cherrypick the commit and it will work) - any other reasons?
It's documentation written when GOPATH was still a widely used for development.
I think nowadays it's less of a concern to have a module first code base with GOPATH unsupported. It's nicer to use a V2 branch. Or delegate V1 to a V1 branch and maintain V2 in master.