I know for certain back in 1.12.2 mobs were able to spawn in placed string. I was testing a mob farm design I had for 1.12.2 in 18w45a and mobs no longer spawned inside the mob farm. I went into 1.13.2 to check that and they don't spawn there as well.

I had the string suspended in the air by 1 block and not on top of a spawnable block. So between 1.12.2 and 1.13 something changed with placed string that no longer allows mobs to spawn inside it. I only now just noticed cause I've been focused on testing the mechanics of newer items being added and forgot to check my redstone builds.