/island command, provided by Bentobox. However, they have no way of returning to the overworld. I could very easily make a command alias for players using MyWorld's command /tpp overworld through commands.yml and Luckperms, but that does not return the player to their last location, but rather, their last location in the overworld - i.e. what if the player ran the /island command in the nether, and then wants to leave their skyblock island? This would be a great inconvenience to a player who had been in the nether. /tp to teleport, they'll be using /island, so /back (from EssentialsX afaik) won't work.