I don't think anyone is pretending that Go is a decades-old language. What is the standard term for this kind of inlining which would distinguish it from the inlining already present in the compiler?
"Go inlining improvements..." and then "Conventionally, inlining includes ... but Go has been limited to ... until recently. With ... Go now supports inlining in a broader array of circumstances."
There isn't a standard term, because virtually all inliners always did what Go is now doing. It's pretty basic functionality.