When the BlockSpreadEvent is used on the results of a Sculk_Catalyst spreading sculk, plugins are given the Source and Block from the event. Source ends up being the newly minted Sculk or Sculk_Vein. The event Block ends up being the previous block that is changing into Sculk/SculkVein: air for Sculk_Veins; another block like Grass_Block or Dirt/Stone when it is changed to Sculk.

Protection plugins would find it very useful to know the true source: the Skulk_Catalyst.

Without knowing the true source, protection plugins cannot prevent sculk and sculk_veins spreading into areas they should not.

This might be a case where Sculk_Catalysts throw a BlockFertalizeEvent, if you don't want to alter how the BlockSpreadEvent is used here.