Skyblock Separator is an extension of https://github.com/MelanX/SkyblockBuilder which separates the players' inventory on a per-island basis and adds an impassable border between islands.
The flagship use case of this mod is a cooperative pack called The Inventor and their Friends, where multiple independent islands complete milestones in order to gain points to unlock their shared tech tree.
You probably don't need me to say this, but since I work there, I need to say it anyway: This is not an officially supported Google product.