[Zeke]

I have found on many old homes (all built before WWII) that what you found is correct. However, that first course of what appears to be shiplap tells me that it was 3 1/2" fir T&G that was sanded to the point that the T&G started to look like shiplap.. If so it's useless.

The modern way to do this, as Trekkor would have said, is to do exactly what is there. E.g., 3.4 ply, backerboard set in thinset, and then tile set in thinset over that, A good solid floor that will last for years.

If that was my house I would (and have) removed it all down to the joists and start over with Trekkor's method. Whether this meets the existing floor level beyond the kitchen is a function of your choice of flooring. Typically the old flloors were sheet goods like linoleum. So no very tall.

In my own 93 YO home, the bathroom joists were cut down 1-1/2" so a concrete float and tile would be flush.

You could get lucky and find that the cement board on you floor was not laid in thinset and therefore will pry up leaving the plywood which should be re nailed or screwed. Squeaks can be handled from below. Tommy on TOH has a few segments on those methods.

I have a job planned for sometime this year where the tils floor is well ve and inch above adjacent hardwood floors in a Monterrey Spanish home of similar age as mine. I have assured the client that the floor will come out in large chunks with little to no jackhammering or saw cutting. She is fastedious.