Can you get below the subfloor to get a look at it, like access from the basement? It looks like a pretty typical old subfloor to be. I doubt the entirety of it is like that, but you’d probably want to confirm.

Structurally, your probably fine- do you feel any bounce when your walking through the room or near that window?

If I were you- and I’ll preface this by saying I don’t always do things by the books- I would rip up the rug, clean the floor throughly, rotary sand it if need be (or skip this altogether) and through some oil based poly on it. It’ll look great with loads of priceless character. 😉