Individual sites/sub-sites should definitely have the option of locking down or at least specifying the engine that they depend on. (In the case of the latter,

Individual sites/sub-sites should definitely have the option of locking down or at least specifying the engine that they depend on. (In the case of the latter, in theory GitHub could collect the “in use” specifications of all projects and get some idea who needs what before making a change.) Ultimately though, if you need control then you have to take control. I have been using "textile" for the help documentation in my project for years. I simply copied the entire Python rendering code into my project. Nothing can break until I am ready to change (and I rarely find new versions of the rendering engine anyway).