These are mainly single user (as in having one user instead of having multiple User ID) operating systems. Some may have limited, or newly added multi user functionality: such as some are single user for their desktops. Security for logging in and complex permissions aren't typically needed on single user operating systems.
Maybe: https://github.com/minexew/Shrine.
I missed some, but feel free to add them. These may be some of the more modern ones. Other operating systems need attention. Not sure about SvarDOS.
- Haiku is a desktop OS for x86 computers. It has lots of newly developed features and replaced programs, even though development seems to occur slowly.
- RiscOS is a desktop OS for 32bit Risc architecture. It relies on a lot of GPL programs. They're making progress on its own suite of desktop applications. For 32bit Risc, this may be the best lightweight OS to use. When you're looking for a simple lightweight desktop on a RaspberryPi or BeagleBoard, that doesn't store confidential files, this is the one to use.
- PDOS: Public Domain OS or Public DOS. DOS is known for being a single user Operating system. This OS is available on x86, 16bit Risc, 32bit Risc (it runs full screen programs in place of Windows, as DOS otherwise typically maxes out at 16bit) and other architectures.
- Minix gets a mention, even though it's defunct. For learning purposes, it's not defunct, while for most other purposes, it is. It's for x86, and the only Risc board it has support for is Beagleboard. Minix also uses BSD init for its initialization style.
Maybe: https://github.com/minexew/Shrine.
I missed some, but feel free to add them. These may be some of the more modern ones. Other operating systems need attention. Not sure about SvarDOS.