How would I detect a player who left the game and rejoined to see if they clicked a proximity prompt

If u only have the boolean to save then u can do
local dataStore = DataStoreService:GetDataStore(“datastorenamehere”)
local success, err = pcall(function()
dataStore:SetAsync(player.UserId, data) – this is the true or false
end

then use getasync to get the value when the player joins.