Did I get it right ?

Extension is an extension, although you may call it add-on, option or whatever else. It is something that is added to a core functionality. So what does this 'deprecation' means ? Literally 'we at PHP development team do not want you to add this extension', isn't it ?
Well, but why shouldn't we ? Is it poisonous or what ? It is just a bunch of functions, and there is one important thing about mysql_ - it works.
If it works, don't fix it.

Lately i ma using postgresql, i think, Pg is better(but mysql is faster) but in syntax is only a little diference. So have fun MySQL users.