You go through security as you go airside. Then when you get to the gate waiting area you go through a second security. At this point they take your water and there is nowhere to refill in the waiting lounge so impossible to take onto the plane. I traveled through KL in February 2019.
There is one near immigration next to a map of the terminal and there are a number of water fountains all around the airport. So basically once you enter the airport in immigration you will need to empty your bottles, then you can fill them until you board.
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