Is it correct to put plywood outside front door?
Nope, it's not normal, but it isn't necessarily incorrect. Actually, you can see the ends of the original planks under the plywood...just above the old rot in the top stair riser and next to another plywood patch. However, it can continue to work if polyurethane or paint were maintained.

There is pressure-treated plywood for exterior use that could be used when this rots out, if you like the look. But again no, traditional natural exterior grade wood is the only "correct", but many today even go with backyard deck type pressure-treated planks (not panels).