I don't know if I would consider cartridges that are still produced, even if only in "seasonal" runs as obsolete, but the word has different meanings and uses depending on context.
Some folks consider the revolver obsolete. I have actually heard people say it.
The way it is usually phrased is the ______ is obsolete for ________ use.
This applies to many things, sadly, myself included.
Nostalgia has revived many "obsolete" rounds. Wish it could do that for me..
I'd pick .45-70, it has been "obsolete" since the 1890s, but is still in production today, possibly at higher levels than anytime in the past century!