I've caught slime spawning through slabs at regular full block height as well as on half slabs. My base is half slabbed everywhere to prevent mobs from spawning, mostly slime because it's underground. So far, I've only experienced this on the bedrock realm I play on. I have not tested it out on singleplayer worlds or other realms/servers. The realm has a nether hub for fast transportation and every now and then, when I go through my portal to the nether, I have mobs such as spiders, skeletons, even creepers waiting to blow me up and by the time the dimension buffering screen goes away, they're already ready to explode/attack.