I would not use treated wood. The salts will eat up any metal. The original floor is 3/4". If the area is relatively small. Cut out the rot far enough to get into good wood on the old floor. Then cut some 3" wide strips of plywood, slip them under the edge of the opening where they provide a support for your patch, glue and screw the strips to the old floor then insert your patch to cover the hole. The patch will rest on the strips you installed, then just attach it with screws.
The chances are good that part of your patch will rest on the outrigger for the steps if the rotted area is where I think it is.
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