Always fun to see Multics pop up; the influence it had on computing is pretty impressive and its influence lives on in many projects. As just one personally relevant example, the SCOMP mentioned in the glossary [0] and described in more detail on the history page under 5.4.1 [1] became the STOP operating system which is still in active development and is what I still work on today. (Technically, the SCOMP was the whole machine, and STOP "SCOMP Trusted Operating Program" was its operating system). Up until pretty recently, we still had a Multician working on STOP, and have a guy from the Honewell days still plugging away on it.
I've been gainfully employed for well over a decade working on it and it's been around in one form or another for over 40 years. We're constantly improving performance and capabilities, adding support for more hardware, supporting the specific needs of our customers etc... Just like any modern operating system, it's never really "complete". STOP is a "security from the ground up" OS, where security isn't just a first-order priority, it's the entire point, typically used in/as multilevel security solutions.
There's a link in my profile to the company products page for my group, which includes a link to the STOP OS page. There used to be additional documents you could download from those pages, but it looks like they're not working any more.
The short version is that it implements three different MAC (mandatory access control) policies (RBAC, Bell-LaPadula, Biba) and the standard *nix DAC policies. It's designed for safely handling/moving data on/between multiple classification levels. (See the SCOMP section in [0] for history). From a user perspective, it's very similar to Linux, with a largely Linux-like ABI and similar user interfaces, including a full X/xfce GUI environment if you want, though most actual deployments tend to run headless with only required software loaded. It runs on both small embedded boards and large enterprise servers and a bunch in between.
The data diode one reminds me of a null-modem cable I once did where I forked the TX line to a second DB-25 so that a server could eavesdrop the data coming from the PABX to the call tracking box. The server would then push it to all stations connected to a socket, where a Java applet would display the proper greeting the support agent would use when the call came in.
[0]https://www.multicians.org/mgs.html#SCOMP [1]https://www.multicians.org/history.html