Cutting into the trunk would not likely be life threatening to the palm but would do damage and would create a weak spot in the trunk. Therefore it is not advised. If you can trip back the pool decking that might be an option. Otherwise you might want to consider having the palm moved a little away from the deck. This is a pretty big job but would be a long term solution. If you have a certified arborist come to your place they could see the situation firsthand and may be able to see another option.
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