Thread:Project:Support desk/Writing extensions - where is platform's API?

Hi everyone! This question is NOT about MediaWiki API I'm writing MediaWiki extensions for some time and still can't figure out what parts of the platform are guaranteed to be changed slowly and carefully. What classes can I rely upon? What classes and functions are less likely to change in the new versions of MediaWiki? The database structure looks pretty solid but I try to install old extensions often see the errors that some functions haven't been found.