What it says on the tin.
It's already been seen that the servers have this configuration, though maybe empty, and this update more than any other update makes sense to actually implement it.
My original idea was to tie Broodwatcher Unlock to the lack of this tag... But that's pretty unlikely to go over well especially with clans die to the issues that can cause (even tho not killing rival hatchies reduced full-clan wipes and elongates life of PvP but oh well lol).
I think it should at least be detectable. If not tied mechanically, if no actual disadvantage is given to hatchie killers, it should be able to be seen by other players to know to avoid them.
A scent icon was my first thought but how often would that be used? And in flight, impossible. So maybe some aura affect, or... like how the isle has random biting brain disease for cannibals? Something visible, something noticeable to tell me who to target, but not necessarily something that actually puts you at a tangible disadvantage.
Anyway yeah, given BW is so hatchie oriented it would make a lot of sense to clamp this into the update, or maybe a patch update shortly after if time is not looking good.