It seems the OP is asking about which cartridges
have died out -- not which ones you speculate
might die out.
I don't think fans of the 7-30 Waters, or certainly of the 6.5 Grendel would agree that they've perished from the earth just yet.
There are plenty of cartriges like the .30 Remington, or the .401 Winchester which have more or less completely gone away.
Sort of.
With the increase in diversity in the reloading world these days, most obsolete calibers still live on in the hearts of their fans. Heck, I'm looking at buying a rolling block Remington in .430 Spanish. Blackpowder cartridges like that sort of define the concept of an extict cartridge, but they certainly are still popular!
(And, some of the evolutionary dead ends seem to show up in the darnedest places. The .30 Remington was used as the parent case for the 6.8 SPC. The .221 Fireball is the actual parent case for the .300 Whisper and clones.)
Places like Old Western Scrounger and RTG offer a more complete list of REAL hard to find stuff than you probably ever knew existed.
Check out RTGs list:
http://www.rtgammo.com/obsolamm.html
Play around on OWS's pages for some real mystery numbers:
http://ows-ammo.com/store/index.php?main_page=index&cPath=58
The only ones I trust to be
really dead are the various large-caliber rimfires. .25 RF, .32 RF, .41 RF, etc. No one's making them and they can't be reloaded. That's about as extinct as it can get.