Not sure if RPG use would be covered (it needs some delving) but this could increase your art options for projects

however you'd need to be 100% sure there weren't usage exemptions or potential problems (as may apply to some art with people in etc) and so on (and that the work was out of copyright in the first place in some cases) and adhere to the CC0 rules especially:

• In no way are the patent or trademark rights of any person affected by CC0, nor are the rights that other persons may have in the work or in how the work is used, such as publicity or privacy rights.
• Unless expressly stated otherwise, the person who associated a work with this deed makes no warranties about the work, and disclaims liability for all uses of the work, to the fullest extent permitted by applicable law.
• When using or citing the work, you should not imply endorsement by the author or the affirmer.

If anyone has any relevant legal knowledge does this mean such work being acceptable to use as RPG artwork?

https://kottke.org/20/01/paris-museums-put-100000-images-online-for-unrestricted-public-use

http://parismusees.paris.fr/en/actualite/open-content-150000-works-from-the-museum-collections-of-the-city-of-paris-freely

https://creativecommons.org/publicdomain/zero/1.0/

http://www.parismusees.paris.fr/sites/default/files/medias/fichiers/2020-01/CP Paris Musées Open Content EN.pdf