Here are the steps to reproduce the glitch:

1. Place inverted stairs or slabs [As shown in images]
2. Place water inside them and put slabs so it doesn't flow (It doesnt matter if it flows) [As shown in images]

And mobs wont be able to jump over them, flying (Ghasts,blazes) jumping (magma slimes, slimes) and climbing (spider, cavespiders) mobs will still be able to jump over these blocks.