I just checked a wide variety of websites of paper conservationists/restoration services online
They all say they have done work for galleries, museums, collectors etc.
They offer variations of retouch, reconstruction - filling, mounting, cleaning and treatment of oversized work
Here is an interesting one
http://oliverbrothersonline.com/our-work/recent-clients/They work for a lot of institutions according to their list...with a focus on paper
And that typed letter on the page is telling...
My gut feeling is Museums do a lot more restoration outsourced than we think...and who knows what that restoration is
since museums are operated and run with varying degrees of professional oversight...depending on the restorationists they use ...i would stay away from absolutes when it comes to their restoration practices...restoration is constantly evolving after all
Can we safely say we don't really know for sure how much paper is restored by museums in their collections and what the extent of restoration really is??