RE: Are there exist any registry for reserved key binding?

[Drew Adams]

> > (elisp) `Key Binding Conventions'
> 
> So any major/minor mode designer MUST follow this rules.

No, there is no MUST.  There is no code-convention police.
Judgment is by code users, not tribunal.  Users include individuals and other
code (e.g. libraries).

The reason for such conventions is to help make it easier for different bits of
code to get along together peacefully.  And to make it easier for users to use
such code.

> 'jabber-keymap.el': So this package define 'C-x C-j' globally.

Did you see any guideline at (elisp) `Key Binding Conventions' that says please
do not bind a key in the `C-x' keymap?  AFAIK, that is not proscribed.

> If another package use this key binding user must resolve
> conflict manually?

Guess so.

If you find what you think to be a mismatch wrt the suggested coding conventions
(they are guidelines, not enforced laws), consider reporting it to the code's
author/maintainer as a possible bug.  S?he will most likely appreciate it, if it
truly goes against the guidelines.

> Are there any guide to avoid conflicts?

Just the suggested conventions, AFAIK.

> My suggestion is to check most popular packages and make some 
> thing different key bindings. But globally maintained key
> binding registry make life much easy...

Consider sending your suggestion as an Emacs enhancement request, via `M-x
report-emacs-bug' (that is for enhancement requests, in addition to bugs).  Or
bring up your suggestion on the Emacs development list, emacs-devel@gnu.org.
(help-gnu-emacs@gnu.org is for help with GNU Emacs.)

But keep this in mind, if it helps: Emacs has been around for 35 years or so.
This kind of thing has been dealt with by thousands of users and code libraries
over the decades.  That's not to say that you might not have a better idea, but
it might give you some perspective on the problem you feel you've discovered.