In 1.14, all blocks through the entire height of the world are used to calculate spawn chances per block. The more blocks piled above or below the platform, the slower the spawn gets because there are more blocks to choose from (even if all the ones outside your grinder would fail the check). Additionally, 1.14 seems to be including the transparent blocks as well, which means blocks like glass, trapdoors, sea lanterns, redstone lamps, glowstone, etc are now considered spawnable blocks (again, even though some/all would fail to produce any mobs for other reasons like light level or being the wrong block type).
How high up in the world you build it will also impact how fast it produces mobs, which is something that was in effect in 1.12 and earlier. In earlier versions of 1.14 (may have been fixed as of 1.14.2), bugs related to the horizontal positioning of the platforms also impacted how fast mobs would spawn...platforms built in the negative directions would not spawn mobs at all/spawn very very slowly, whereas those built in the positive directions would spawn mobs really fast. Additionally, platforms that were placed entirely within chunk boundaries would spawn mobs fairly quickly because there was a greater chance of pack spawns staying within the chunk versus crossing chunk boundaries (the pack spawning crossing boundaries would prevent the entire pack from spawning).
Finally, 1.14 introduced bugs to the spawning algorithm that caused unexpected behaviors, like the player having to physically move around in order for any mobs to spawn at all. Some of these may still have been present in 1.14.2.