Window shading, to borrow from the KDE documentation:
Shade, on the other hand, causes the window to be reduced to simply
the titlebar. Double clicking on the titlebar again, restores the
window to its normal size.
On MacOS, it was reportedly first available as an extension, and included in MacOS 8, but then disappeared in MacOS X.
Usefulness of window shading
While users have commented on the usefulness of window shading, that has been
mostly in the context of full windows.
My primary desire to have window shading is to use it with an OpenStreetMap editing application, JOSM. It can have floating image windows that I do not want to dock (not enough screen space), and want to see those windows every now and then. On KDE/Linux, I can doubleclick the titlebars to reduce those floating windows to the titlebar only, and then doubleclick them again to see the image.
Moving the windows around is a very poor substitute, it is very inefficient. While individual applications could implement hacks to imitate window shading, that is an excessive work on the application development side (which is not too likely happen with JOSM in particular).
Window shading in MacOS
As mentioned before, window shading was an internal part of MacOS, but got removed. While there have been some 3rd party applications, they suffer from functional problems as MacOS is updated or do not offer proper window shading experience.
With Apple notoriously ignoring user feedback, return of the native functionality seems not too likely.
Are there any good, free solutions for window shading in MacOS Mojave?
Although you said free, there is a paid app that seems to offer a similar feature.
Deskovery claims to "Minimize your windows to their titlebars".
I haven't used it.