Since people here are passionate about playing old games on modern hardware, probably some folks will be also interested in "modern-retro" operating systems.

Here is some cool projects:

SerenityOS
SerenityOS is a love letter to '90s user interfaces with a custom Unix-like core. It flatters with sincerity by stealing beautiful ideas from various other systems.

Roughly speaking, the goal is a marriage between the aesthetic of late-1990s productivity software and the power-user accessibility of late-2000s *nix.

SerenityOS has amazing community that is quite big and very friendly for such niche project.
ATM it's is not a very gaming oriented OS, but it's getting better with things like working ScummVM and active development on hardware acceleration and future Wine support.

https://serenityos.org/
https://www.reddit.com/r/SerenityOS/

You can watch videos of the system being developed on YouTube:
https://www.youtube.com/andreaskling

Haiku
Haiku is an BeOS (https://en.wikipedia.org/wiki/BeOS) inspired operating system under constant development. Specifically targeting personal computing, Haiku is fast, efficient and extremely lightweight.
Currently it has several homebrew games and even functional RetroArch and DOSBox ports.

https://www.haiku-os.org/
https://www.reddit.com/r/haikuOS/
Review: https://www.youtube.com/watch?v=UhRQ9OkISqM

AmigaOS
Amiga (https://en.wikipedia.org/wiki/Amiga) is a living legend.
It has a huge and undeniable impact on gaming history and personal computing in general.
And believe it or not this platform still receives new software and hardware even today.

Amiga Story: https://www.youtube.com/watch?v=ws3DJF7MbMU
Top 100 Amiga Games of All-Time: https://www.youtube.com/watch?v=tWQfX-hP1RQ
Personal Computing On An Amiga In 2021: https://thedorkweb.substack.com/p/personal-computing-on-an-amiga-in-2021

https://amigaos.net/
https://amigaworld.net/
https://www.amigaforever.com/
https://www.reddit.com/r/amiga/

BTW, You can emulate it on PC: https://www.youtube.com/watch?v=4Vw-tkTG4Fk

Aesthetics bonus

Chicago95
XFCE / Xubuntu Windows 95 Total Conversion.
https://github.com/grassmunk/Chicago95
Review: https://www.youtube.com/watch?v=j5UeSj2F-yc

Not so Common Desktop Environment (NsCDE)
NsCDE resembles CDE (https://en.wikipedia.org/wiki/Common_Desktop_Environment) look (and partially feel) but with a more powerful and flexible framework beneath-the-surface, more suited for 21st century unix-like and Linux systems and user requirements than original CDE from early 90s.
https://github.com/NsCDE/NsCDE
Demo: https://www.youtube.com/watch?v=ufcSwmCIdMk

Snowdrop OS: 16-bit free and open source operating system.
http://sebastianmihai.com/snowdrop/

Amiga OS 4.1: from what I could find, it's currently owned by Hyperion Entertainment, and it can be purchased either directly through their site, or through official third party sellers.

Amiga OS 0.7 to 3.X: not really a modern OS made to be retro, but the system files can still be purchased as part of the Amiga Forever bundle.

If you have Amiga 500+ or Mac PPC, take a look at MorphOS (https://morphos-team.net/intro).
Some history behind it: https://www.youtube.com/watch?v=DcjW4q_8Eo0