The reason interface smuggling exists as a pattern in the Go standard library and others is because the Go team (and those who agree with its philosophy) take breaking API changes really seriously.
It is no small feat that Go is still on major version 1.
I can't call it a design oversight no, because I'm not sure what reasonable alternatives were considered before Go v1 was released. I also don't have context of all the decision factors that went into Go's spec. To be honest, I'm not anywhere near an expert on programming language design - I'm probably the wrong person to ask.
I am thankful that they haven't broken the spec to change that design, but maybe others don't care about that as much as I do.
It is no small feat that Go is still on major version 1.