It's included as internal on netstandard/net472, and the C# compiler may bake a reference to that into a consumer. If that consumer is then used with a net8.0+ build, the IsExternalInit needs to be there and forwarded to the real one.
This switches our polyfill files to always be included but the actual contents ifdef'd out on TFMs that already have the contents. That then makes it easier to do specialized ifdef'ing in the future, as this does for IsExternalInit.
