I realize this post is about a year old now, but it might be worthwhile updating the documentation here:
[Details Panel Customization in Unreal Engine | Unreal Engine 5.2 Documentation][1]
It describes customizing the details panel similar to the OP, using ILayoutDetails, and has an (Unreal Engine 4.9) tag on it.
It looks like someone started a more current guide here:
[A new, community-hosted Unreal Engine Wiki - Announcements - Epic Developer Community Forums][2]
But I would like to know how to do the things described in the pictures of the original documentation using IDetailCustomization rather than ILayoutDetails.
For example, is there an equivalent for Category.BeginLine() which allows you to stack properties horizontally on a single line? as shown here…
Thanks!