The client-side developer can easily track bestiary info through the tablist, but it requires this widget to be enabled. It also could not effectively track kills past the max bestiary milestone.

Trying to faithfully attribute a mob kill to a particular player has proven to be not viable on the client as it stands. Magic damage and loot/bestiary kill sharing are the main culprits that required mods that wanted to provide cosmetic features for bestiary to be constantly checking against the profiles endpoint. (assuming they don't just use the tablist and deal with the consequences of such)