Pacifiers can delay a child's speech when they have the pacifier in their mouth constantly.
When a child is constantly sucking on a pacifier during the day then they have trouble learning to talk because the pacifier gets in the way.
A child should remove the pacifier when talking and you should not let the child suck the pacifier most all of the day because they need to be able to talk without the pacifier.
Once a child gets to 18 months old the pacifier should really be removed at least during the day so that the child can learn to talk without the pacifier being in their mouth.
I see some kids of 3 and 4 years old still using pacifiers and that's okay but it makes it harder for them to speak.