Minecraft v1.14.4
Forge v28.1.116
Quark r2.0-209

Can the block and item tag #forge:supports_conduit be added to prismarine vertical slabs as well as all the elder versions?

Not the best idea right now, because the way Forge decided to add it might be getting changed really soon, see MinecraftForge/MinecraftForge#6438

Huh. Good point.