What this bug basically allows for, is passive mobs to spawn on none grass blocks, such as stone, concrete, beacons, iron blocks etc etc. The only thing the game is really taking into account for where to spawn things, is biome.
So for instance, in super flat river world, with absolutely no water at all, you can have drowns spawn on land (as long as its part of the river biome). As another example, guardians can spawn in air. As you can see from these couple examples, the block check mobs should be doing is clearly missing in recent updates.
As a side note on drown spawns, they are the ONLY hostile thing that will spawn in river biomes, water or not. Just an interesting thing i noticed while doing some testing
You can see a full demonstration and description in my in my most recent bugrock of the week episode: https://youtu.be/595uxUwfYaE
Update: Since 1.10 animals have stopped spawning on none-grass blocks, but now things are spawning on slabs (realms only) and also spawning in high light levels (may also be realm only), so one step forward and step back on this one. Otherwise drowns can still spawn on land, and everything else mentioned here seems to still be accurate for 1.10/1.10.1