emacs key bindings reserved for the user C-c [a-zA-Z]

[Ali Tofigh]

I read that part of the emacs key binding convention is to keep C-c
[a-zA-Z] reserved for the user and that packages should not make use of
such key bindings. There is also a note about all major modes having
been changed to comply with this convention. I'd like to know:

1) How long has this convention been in place and when where the major
   modes changed accordingly?
2) To what extent do packages actually abide by this convention (at
   least the ones that are actively being supported/developed)?

/Ali