I don't quite like the word "private" because it seems to indicate that
you would want to "forbid" the user to use some methods.  That's not the
case. 

The word "hidden" better describes it, since it is more because I myself,
the person who writes some objects, would like to hide some methods. 

If a user turns the flag "show hidden" to true (perhaps even globally)
then I don't care about that, since then the user explicitely chose to
see all methods of all categories (whether the category name starts with a
dot or not).

To compare with a Unix file browser, a Squeak category is somewhat like a
Unix directory. 

A "private" directory is one for which I do : chmod 700 mydir

A "hidden" directory is something like .mydir

Depending on the "mode" of the browser, .mydir and its contents may be
hidden, or visible like any other directory ... 

The point of hiding is not necessarily to make it secret, or difficult to
use.