In a nether farm, sticky pistons pushed blocks into the space where pigmen's heads were. It used to suffocate them in 1.7.9, but does not in the current snapshot.
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My apologies, there is also a half slab under where the pig's stand. So there is a half block space above their head. This used to suffocate them in 1.7.9 though and does not anymore.
Suffocation mechanics changed in 1.8 so that instead of mobs suffocating when a block overlaps with the top full block their hitbox, it's only the top half block. I would argue this works as intended, since for most mobs (including Zombie Pigmen) their head appears to occupy the top half block of their hitbox. If you wish to duplicate the previous functionality of a 1.5 block tall crusher (killing pigmen but not wither skeletons), use a full 2 block tall crusher instead.
Is this still a concern in the current Minecraft version? If so, please update the affected versions in order to best aid Mojang ensuring bugs are still valid in the latest releases/pre-releases. If this has been done, we can reopen the issue.
Keep in mind that the "Resolved"-Status on this ticket just means "Answered", and that we are waiting for further information on whether this issue still exists or not. We will reopen it as soon as the requested information has been delivered.
Unable to reproduce.