Spiders are now spawning in areas slightly larger than 2 blocks. Previously, if you were to put down cobblestone fences to stop a flat area from having any areas larger than 2x2 to stop spiders from spawning, it would do just that. But now, either the spiders have changed in hitbox or spawn requirement size, or they are completely ignoring the top 1/2 block worth of the cobblestone wall.

Now, it is worth noting that wooden fences are thinner, and spiders would still spawn with those in place in previous versions. However, 1.13 and 1.13.1 have introduced this issue with cobblestone walls only (so far in testing).