I'm a bit broader in my definition, in that a game's dead when
1) No one's heard of it/there isn't a community for it
2) No company is making product for it
3) No one's picked up the ball to produce ongoing content
People can still be playing it if it's "dead", but if noone's making content for it, that's a good indication the community's dead and anyone whose playing it is single group or three.
There's several RPGs I own (and sometimes play) that I'd qualify as "dead" - Alternity, Boot Hill, Gangbusters, Fading Suns, Spycraft, Star Frontiers, TSR Marvel FASERIP, Gamma World, FASA/Last Unicorn/Decipher Star Trek, WEG Star Wars, Rolemaster & Middle Earth Roleplaying (by Iron Crown) - just to name a few.