About a year or two ago there was some extensive discussion on the Forum about how people used the different layers. The Footprint Library Conventions document (KLC) gives you moderate insight into how the development team imagine the layers will be used. Many of us have developed our own practices for layer use. Inertia and old habits are difficult to overcome.
I put this kind of information on the "Dwgs.User" layer. This keeps all the details about the board in one place (the KiCAD file), for easy reference when you place the NEXT order, or revise the design documentation, etc. I also place selected dimensions on that layer. It reduces the time spent answering questions like "Was this board done in 1 oz copper or 2?", "What plating material did we order last time?", etc.
The key outline dimensions are also on this layer. That's an old habit going back over 20 years, when board fabricators almost always asked for a dimensioned sketch, preferrably 1:1 scale, of the board. It's also useful when working with the folks doing the packaging and mechanical design, where various forms of the question "How big is this thing?" arise repeatedly. (Cutting out the outline, after printing to a reasonably accurate 1:1, gives a paper-doll puppet that you can hold in your hand, push around inside a chassis, check for alignment with other features, etc.)
The "Notes" from this layer typically get duplicated in the README file I email to the board fabricator, inside the *.ZIP folder containing the Gerbers.
I use the "ECOx.User" layers for assembly information, including cartoon-like sketches of the components that help visualize the completed assembly. This GHOM hasn't yet taken the leap into 3-D modeling. My "Margin" layer carries information about mechanical interfaces, such as outlines of the enclosure where the board will mount, locations of front-panel controls and connectors, projected outlines of places with height restrictions, etc.