[1] https://creativecommons.org/2020/01/10/paris-musees-releases...
The downloading and re-use of medium-format photographs published on the collections website representing works that are not protected by copyright (hereinafter called the “Photographs”) are permitted, free of charge, for any non-collective use within a strictly private context and for the following exhaustively-listed museographic, scientific and educational purposes: - projection and distribution for the purpose of museographic, pedagogic and scientific activities, such as their reproduction on labels and exhibition signs, the presentation of guided tours, the running of educational workshops, the delivery of teaching and training sessions and the holding of symposia and seminars; - publication of exhibition and permanent collection catalogues, scientific papers and Ph.D. theses for publishers whose registered office is in the European Union, within a limit of one thousand five hundred (1500) copies, republication included: - digital scientific and educational publications.
It is more permisive than I expected it would be, frankly.
So my understanding is that it is the photographs of those works that are copyrighted. In my view it goes against the spirit of public domain to use this in order to restrict the use of the collections put online.
However, I think the laws for this vary quite a bit across countries, and in many countries the photograph is considered a new copyright work. In general it's pretty frustrating how many legal barriers there are to accessing and reusing old works of art. Thankfully a growing number of museums have made things easy with clear copyright releases (Rijksmuseum, Paris Musées, the MET), but others seem more interested in preserving their ability to sell prints.