Plumping is just injecting saltwater into the meat after it has already been killed. And that’s fine, as neither salt nor water are forbidden in Islam. The only thing that could matter is that it’s clearly labeled on the packaging how much percentage the meat is vs how much saltwater was added, as you’re not supposed to be deceptive.

But considering some countries generally lax product labeling laws, I assume you could still get a greyzone, where it could be labeled as “halal-slaughtered chickens” - but the actual product wouldn’t pass a stricter control due to undisclosed brine.

So it’s also likely you have a good grocer, but just know that you can’t automatically trust some halal-stamp if you want to avoid plumping.