If you're talking about adding a "miter slot", no, this will not weaken the table at all. The depth would need to be equal to the thickness of a miter slot bar which will need to be added.
If your wanting to add feather boards, these can be clamped directed to the table without interference from them quite easily and without adding a miter slot. You can even make a simple fence that allows you to clamp feathers boards to them as well. However, most fences anymore have slots already in them.
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