It worked in prior versions but in 1.21.1 it stopped working. It's unknown why it breaks in this version as it's not in the change logs.
This is the command I used to get a zombie that will call reinforcements
/summon minecraft:zombie ~ ~ ~ {Attributes:[{Name:"zombie.spawn_reinforcements",Base:1.0},{Name:"generic.movement_speed",Base:0.0}]}
Comments 3
(Or, in 24w33a:
/summon minecraft:zombie ~ ~ ~ {attributes:[{id:"minecraft:spawn_reinforcements",base:1.0},{id:"minecraft:movement_speed",base:0.0}]}
)
The correct command is: