Idea:
We should add a field in the registry for to mark packages as "unlisted".
This can be applied to abandoned packages, or to packages that gave been deprecated or renamed.

The effect of this woud be to make it so that pkg> add ... REPL does not tab-complete to that name.
Possibly some other things might like to use it, e.g. JuliaObserver, or JuliaHub might do something with it.

To go beyond this, perhaps we could have a free text field that the unlisting reason could be given.
Which would be displayed when people directly call pkg> add.
And that could give a warning message explaing e.g. that it was renamed and that other one should be installed instead,
or that it has been discontinued and a good alternative might be...
Such a message would not be displayed when it is added indirectly.

Naming things is hard, so perhaps this should be listed = false or visiblity = supressed,
or even unlisted-message = "Reason this is unlisted" and if not present then it is listed.
or something.