This is nothing new. The reduced XP has been in the game since the start of classic 2 years ago. If anything it’s been changed to be more lenient to boosters. For Classic the xp nerf came if you went 15+ seconds without damaging a mob, not 40. So maybe thank blizzard if it’s actually Been changed to 40.
The way it works is based on a mobs remaining Hp percentage when you stop attacking. Say you damage a mob to 25% HP and the. Stop attacking for 40 seconds, the mobs total XP available will drop from 100% to 25% causing you to miss out on 75% of the XP.
But let’s be real. Why would you ever go 40 seconds without damaging a mob? The mana rule is 5 seconds to get mana to begin regenning at normal rates. 2 or 3 ticks should be enough for at least 1 spell at the very least.