@HugoC So we did additional tests and searched through Skatter’s archives and… It seems that Maxwell has never supported Render Only mode!
If you search for videos and tutorials that describe working with Maxwell+Skatter, they often mention that it only works in In Model mode. I think that there was some plan to support Maxwell in the past but it never panned out. Sorry for the confusion, we thought it worked too 
We’ll still contact the Maxwell developers to see if they are interested in supporting Skatter.
Yes, it works with the renderers listed on this page: V-Ray 5, Enscape, Thea, Octane, Indigo, Shaderlight, IRender nXt, Twilight Pro, Raylectron, and Kerkythea.