Stop spreading this misinformation. /who NEVER allowed you to determine what layer you were in, or who was in your layer. Blizz even stated this.
Another popular theory is that you can determine your current layer by doing a /who in a capital city, and comparing the results against the /who your friend does. That doesn’t work because /who returns results from the entire realm, not just from your layer, and if the result set is too large it truncates the results before sorting them. This means every player gets different results, but those differences in result set have nothing to do with which layer you’re on. This has led some people to claim that they’ve discovered dozens of layers per realm, but that claim is completely false .