Does this mean that certain parts of the compiler, like the C++ parser, will be removed soon?
No, this just means that parts can be replaced 
@Douglas_Gregor may know some details on the parser.
Is there a documented policy for which host compilers will be supported going forward (e.g., "Swift 6.X can be built with a Swift 6.(X – 2) compiler" or something similar)?
not yet
cc @etcwilde