As far as I know, it's absolutely possible to keep stool (or any) samples for 24 years if they're stored properly. Whether the TMPD did keep the samples is another matter entirely. I hope they did.
@Charlot123 is correct, today it should be possible to tell if a person is likely to be vegan/vegetarian by looking at such a sample. It's more than just what kind of food was eaten during the last meal, a vegetarian diet results in the body producing different enzymes and growing different bacteria in the gut. The sample would contain traces of all those things; today, a pathologist could be reasonably sure what kind of diet the killer was consuming long-term, not just over the past day or two. I don't know if the investigators could have been so sure back in 2000.
A vegetarian potentially committing such a bloody massacre. That would be one for the psychologists to figure out.