I read on the Khan Academy Github page https://github.com/Khan/khan-api that the api will be deprecated by July 2020.
My questions are:
I'm not sure about the Khan Academy API being removed thing, but to answer your second question:
Yes, you can use the API that Khan Academy is built on for personal projects. It is slightly different than pure Khan Academy code, but most of it is from the ProcessingJS library. See this article for information on how to write Processing JS code for personal projects.