Then you have been misinformed.
Alt and tabbing out of fullscreen paused some in game timers.
So achievements that used timers, like the Dalaran mail sorting, could be paused by simply tabbing out, then you go google the answer, tab back in and complete the achievement.
Now you see why Blizzard refuse to answer any questions about it.
But, I’m sure they will put it back when they find a way around the exploit possibilities.
Anyone saying Vulkan/DX12 can’t support fullscreen, need a slap.