That would be true if it were common for applications to directly support alternative UI modalities, but that's not how accessibility generally works. An application implements a GUI, including support for the platform accessibility API (hopefully with the help of the GUI framework), and it's up to a separate assistive technology, such as Narrator (for screen reading) or Dragon NaturallySpeaking (for speech input), to use that generic accessibility support to adapt the UI for a specific need.
So if any assistive technologies are using .NET Framework, they might have a bit of difficulty converting to .NET Core. But that's a small number of applications, and in my experience, most Windows ATs use native code anyway. And using SAPI via COM interop isn't hard; you don't have to get down and dirty with P/Invoke or anything similarly low-level.