[vetinari]

The support library allows you to use features of newer system releases on older systems. See also GDI+ for Win98/NT4 or Windows feature packs for example.

The API changing game is another problem - it does not allow you to use existing application with newer system. As far as I know, this does not happen on Android (for now).

[pjmlp]

There already a few deprecated API, which so far have not be removed, but until when?