Like I say, news to a) my Shia Twelver family, and b) the rest of my Shia Ismaili family. YMM of course, vary.
And of course, “generally” is a very forgiving term. 
The only thing I know is that our local Indian restaurants (which are mostly owned by Pakistanis and, in any case, cater to them as well as Hindus) do serve dishes with shrimp.
Judaism had (pretty much) centralized laws back in talmudic times (say, 100 BCE - 400 CE), although there is still some regional variation on technical bits. Islam never centralized halal practices, so there is enormous regional/national variation. Your Pakistanis could be from an area that allows shrimp, or it could be a non-halal restaurant.