Yes, player prefs is probably what you’re looking for. One option would be to create a ‘bootstrap’ level that reads the ‘has been on the island’ setting from the player prefs, and then loads either the island level or whatever comes after it, depending on the value of the setting.
Have an exit from your island level, which takes you elsewhere, but no triggers or methods of returning once they’ve left. So they can leave, but never return. Unless they start a new character, who’d start on tutorial island.
Edit: I don’t think PlayerPrefs would be a good idea for an MMO, other than to store game settings perhaps. But certainly no character related information.
Like I said, a simple collision and load a new level. How you deal with the rest of that, including placing the character, it’s data etc. will depend on how you’ve got it all working in the rest of your game if this is just a little feature that doesn’t work.
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