Replacing part of an old subfloor in a kitchen due to water damage. Old floor is 1x6 wood slats with plywood on top. Is there any benefit to replacing with slats again? I can't think of any myself, and would replace with T&G plywood. Slats vs. plywood vs. OSB to replace the slats, then another layer of plywood on top of that. I don't know that full thickness of the slat layer plus plywood layer, but if it is 1 1/4" total, I suppose a single thick layer of plywood could replace the slats plus plywood that's being replaced. (The final surface will be tile, so Ditra or cement board will be the top layer.)
I would use 3/4 T X G Advantec with constrution adhesive on the joist, instead of plywood. There's little squares printed right on the panels to show where it needs to be nailed.
Advantec if far more stable, stiffer, and way more moisture resistant then plywood.
Make sure whatever plywood you use as an underlayment is subfloor rated.
That underlayment is going to have to be nailed every 4" on the edges, and every 6 to 8" in the field., it needs to not line up with the seams below, and not be nailed into the floor joist.