Pieces placed from PlantEverything will persist in your world even without the mod installed. They can also be seen by other players in multiplayer who don't have the mod installed. The only exception to this is Ygga Saplings and Ancient Saplings. These are custom prefabs (not vanilla) and will only persist if they fully mature into their tree counterparts first.

There are other mods that do this too. Generally any mod that gives vanilla prefabs Piece components and allows the player to place them should be viewable by unmodded clients.