So in the 1920s, people started finding mammoth bodies with meat so well preserved as to be edible ... if you call half-rotted and definitely freezer burned "edible." Having better food available, and also common sense, the meat was reportedly fed to sled dogs.
But in the early 2000s, Russian scientists weren't as choosy or concerned with self-preservation. Feeling the urge for some sweet, sweet mammoth steak, one reckless soul carved a bit off of the plentiful mammoth carcasses they had in their cold storage and Epic Meal Timed the shit out of it.
The resulting banquet was perfectly edible all right, but tasted "awful" and, shockingly, not unlike "meat left too long in a freezer."
That's not to say all ancient meat tastes like ass, though. When a 36,000-year-old baby bison was found frozen in Alaska in 1979, one scientist knew exactly what to do with it. He performed a bunch of experiments ... and then calmly carved some meat out of it and made a stew, because heavy sciencing makes for a heavy appetite.
The stew was not only edible, but deemed "acceptable." So, yeah, that meatloaf you've had in the freezer since 1994? Some day, after civilization has collapsed, risen again and collapsed once more, some cyborg will heat that shit up and declare it "OK."