At minimum, you're adding a "rewrite your accessibility code" cost to anyone moving from .NET Framework to .NET Core. How many businesses are going to do that, versus say, drop that feature, presumably due to "low usage"? Is Microsoft really serving the accessibility community here? Making it harder to add accessibility to software is going to negatively impact accessibility being available in software.
And if it's just a convenience wrapper, it should be trivial for Microsoft to reimplement it: And it'd be far less wasteful for Microsoft to do it for everyone than expect everyone who uses it for accessibility features to reimplement it themselves.