Minecraft wiki entry for Spawner_traps, associated text: This grinder does not work in 1.13+, since skeletons and zombies no longer swim. However, it is possible to build a similar grinder using soul sand block bubbles to bring mobs to the top of the water column (https://youtu.be/N5m2sReltMc ). See Tutorials/Drowned farming for an alternative simpler design
Note, all dimensions are given the format: Horizontal x Vertical. This method is an indirect hybrid of the aforementioned traps, utilizing both fall damage and mob control with water. In the mob spawning room, dig 1 block down so that the spawner is at eye level, and place water blocks at two adjacent corners. On the wall opposite of the two water blocks, dig a 1×2 hole for three blocks in the middle of the wall (allowing mobs to pass through).