When a mob is added to a Team and the team has a color, the scoreboard teamkill works when a player kill the mob.
Maybe this intended, but this don't have sense if this bug is marked intended: MC-88743
In this bug mobs don't increases the score by killing a player.
Mobs should not interact with any type of scoreboard according to the previous bug.

How to reproduce

/team add test
/team modify test color blue
/summon minecraft:creeper
/team join test @e[type=minecraft:creeper]
/scoreboard objectives add teamkill teamkill.blue
/scoreboard objectives setdisplay sidebar teamkill

Kill the creeper
-> The scorebord does increase, contradicting WAI of MC-88743 / MC-46956