If interested, Timeline of Operating Systems is a good start. Just pick summary of anything major (minus DOS's) and dig into references if it interests you.
https://en.wikipedia.org/wiki/Timeline_of_operating_systems
The capability systems were also forward-thinking with much modern work on OOP, isolation, etc slowly reinventing its methods. Easy extension plus strong security/reliability were their thing.
https://homes.cs.washington.edu/~levy/capabook/index.html
Here's a paper vezzy-fnord shared with a lot of nice examples with features that defined future work and some still ahead of modern systems:
http://brinch-hansen.net/papers/2001b.pdf
Lots of old stuff that still beats modern, mainstream stuff on key metrics. Hopefully, you'll dig into some of it and enjoy the "arcane" stuff yourself. It's arcane, but sometimes awesome too. :)
http://aesec.com/eval/NCSC-FER-94-008.pdf
Note: Mainly see design, layering, and assurance activities.
The most modern use of them is CHERI processor and CheriBSD at Cambridge.
https://www.cl.cam.ac.uk/research/security/ctsrd/cheri/
https://www.cl.cam.ac.uk/techreports/UCAM-CL-TR-864.pdf
Note: Paper references quite a few past uses of them in detail.
A great, modern example in software is Code Pointer Integrity (see 2014 paper link):
http://dslab.epfl.ch/proj/cpi/
So, there you go for tonight. Have fun. :)
It's really nothing special. Copious amounts of time coupled with an interest in system software research that motivates me to read research papers, dive into project wikis and the like.
If you'd like to discuss things further, just reply affirmatively and I'll send you an email.
I've started to read your piece, but I've honestly been interrupted constantly so haven't been able to give it my full attention, which I intend to do as soon as I can!
My disagreement with you over systemd, incidentally, never had anything to do with my assessment of your abilities. It was pretty obvious we both have technical skills :-)