Found Lath and Plaster wall under remodel drywall. Is it restorable?
The closeup looks like cement based backer board used to hold tile in wet/damp locations. If so, that is not original.

It would be very hard to restore to lathe/plaster throughout, especially if sections are missing. The transition points for repairs tend to crack and show lines. Also, wire mesh is used more than wood lath today. Finding a skilled plasterer will be a challenge.

The biggest difference in the appearance between old plaster and drywall is surface smoothness (unless you count cracks and slight waves in plaster). Almost all drywall has a paper surface that is very slightly textured when painted. To get a plaster-like look, you can skim coat the entire surface of drywall instead of just the joints. This is called a Level 5 finish. A discussion of levels of drywall finish can be found here.

Painting also has a role in appearance. Rollers often leave a very slight stippling. This can be overcome by using an additive that improves leveling. Spraying also gives a smoother finish than rolling or brushing.

Once all is done, you only see the surface, so unless this is an historic gem, you might consider top quality drywalling and finishing. If you are working in a damp location, special drywall types are available that are moisture resistant.