MagnaRisa is using a plugin that spawns shulkers in end cities (well technically it turns endermen into shulkers within the bounding box of an end city if they are standing on purpur and are a certain distance from any other shulkers). I built a farm that picks up shulkers with minecarts and suffocates them. It is stackable and tileable.

How it works:

1. Endermen spawn on purpur blocks. Some turn into shulkers, others don't because there are shulkers nearby.
2. Shulkers open, triggering tripwire. A piston pushes a redstone block next to some redstone dust to power the dispenser.
3. A minecart is dispensed and picks up the shulker. It rolls off the edge of the platform and falls onto a sloped track leading down to the killing area.
4. Other nearby endermen are able to turn into shulkers. If tripwire is still triggered by endermen, an etho hopper clock can power a piston every so often to move the redstone block out of place momentarily. This re-powers the dispenser, sending out another minecart to pick up a shulker.
5. When shulkers land in the killing area, they go over an activator rail, which places them one block up and down the track of the rail. This is a solid block so they suffocate in it. The whole area has been cleared of valid spots for the shulkers to teleport to.
6. The minecarts hit a cactus and are dropped into a water stream to return to the top of the farm. The shulker drops elevate themselves up through the blocks into a water stream above the killing spot and are sent to storage.

Efficiency: A minecart is dispensed for each fresh spawn, since shulkers start as endermen.
-After that, efficiency can depend on factors such as length of the track (longer = less efficient because it takes longer for shulkers to be picked up) and clock period (shorter = more efficient because shulkers get picked up more often).
-Decreasing the clock too much may end up with multiple carts on the track, which can bounce back and forth and cause shulkers to clear out less quickly. I have mine set to go off after a minecart has travelled the entire track and fallen down several blocks, to ensure that remaining endermen can convert.
-Usual mob farm efficiency factors, such as height in the world and lighting the area around are also important.

Issues:
-Sometimes an enderman gets picked up. Not a huge deal because it's rare and if it's far enough away it'll probably despawn eventually.
-Sometimes the minecarts bump each other in just the wrong way that one stops halfway off the track and the rest pile up on top of it. Fixed by shooting the bottom minecart, but then you have to go get the minecart or put in a new one. Rare but a mess when it happens.

Improvements:
-The farm would run faster if the tripwire was at the shulker height, not the block above. However, this requires more tp-proofing as shulkers don't like to be in string. I decided it was too much bother for me.
-UncleSamwise made a version where the killing area with the activator rail is in the center, and several minecarts loop around sets of 2 purpur rows and drop off the shulker as they pass through the middle. No need to break and dispense minecarts, no drop where shulkers can get stuck, and endermen that get picked up can teleport back onto the spawning floor. It can however be slower, depending on the length of the track and how bunched together the minecarts end up being.