The implementations for the X Window System are all based on the X Keyboard Extension (XKB), and they all adopt the level concept of Neo 2. To benefit from all possibilities this brings, the XKB part of the implementation is not sufficient. The rich set of available symbols of Neo 2 is in part due to specific dead key sequences. More on the possibilities and the installation is described in the Neo manual (in German).
Since version 2.12 of May 2014, AdNW is part of xkeyboard-config, KOY is included since version 2.17 of January 2016. However, they are contained in the part of exotic layouts. The dialogues of graphical tools for keyboard layout selection need not show such exotic layouts. Fortunately, for his own layout, Marcin Woliński did the work to describe for several systems how to make the exotic layouts visible. In any event, you can select the layout on the command line:
setxkbmap de adnw
The new graphical environment Wayland uses xkeyboard-config as well. Thereby, also the early adopters already using the system should be able to use AdNW without further effort.
The implementation of AdNW adopts the Neo 2 level concept. Unfortunately, every now and then, one runs into programs that have trouble with it. In particular, the editing keys on Ebene 4 do not always work. For some programs, Shift+Space does not work.
For this reason, I offer a modified version of xkeyboard-config (version 2.36), which fixes or circumvents these problems. Furthermore, it contains additional layouts (AdNWzj, Bu-Teck) and a new option cutcopypaste. All the layouts are listed among the conventional layouts. Therefore, they can be selected in graphical tools. Furthermore, I made some simplifications in the infrastructure. This makes the modified xkeyboard-config a good starting point for your own XKB experiments.
However, for X-servers before version 1.12.2, using Gtk3 applications, problems are possible. In this case, use option ebene4_direkt. Presumably, also Wayland will only work with this option.
Finally, there is a ksh script that is capable of creating a file that contains the entire keyboard layout. The script has many options, among others, there is exotic stuff such as a Cyrillic and a Greek AdNW variant. Technically, it is very advanced regarding XKB. Due to the many possibilities, it is messy as well.
This is the solution I use myself. It can be interesting for people working with older systems, or for those that do not use a “Desktop Environment”. However, most people are probably better served by the possibilities described before.
This is a simple implementation of AdNW using three levels for the NetBSD console. With a few changes, it can be reused for OpenBSD. You just have to remove dead_caron and to feed the file line by line into wsconsctl. The three levels are based on Neo, however, not all symbols contained in the first three levels of Neo are available. As the NetBSD console is not Unicode-aware, you should not use it with an UTF-8 locale (such as LANG=de_CH.UTF-8). With LANG=de_CH.ISO8859-1, at least the umlauts and other “western European” characters do work.
In the the download area of the community page, there are other implementations, for the Linux console, for the FreeBSD console, as well as for Windows and for Mac OS X.