This mod changed enemy attacks from "stream" attacks to "projectile" attacks and made them collide with walls.
The release of Factorio 1.1 gave the game new collision layers, so it became more practical to resolve some of the long-standing issues that collision-based projectiles had, such as not affecting flying units. Fixing these issues took the mod into a new direction of changing the way that spitter attacks, bullets, walls, and flying units interact in general, so the name "Walls Block Spitters" did not properly describe what the mod did anymore. For that reason, the mod is renamed "Combat Mechanics Overhaul" and all content has been moved to the mod of that name.
To help with legacy support, if you update the "Walls Block Spitters" it will add "Combat Mechanics Overhaul" as a dependency so you get all of the new content. The last version of "Walls Block Spitters" does nothing on its own other than require "Combat Mechanics Overhaul".