Personally, I love to think back to the times where OS releases were a huge deal for the tech world – that was exciting
https://archive.org/details/PersonalComputerWorldMagazine/PC...
Haiku is also, I'd argue, the highest-profile FOSS OS that isn't some form of xNix. (Waddlesplash will wade in and start arguing now, but that's OK, you can ignore that.)
Haiku is a modern FOSS recreation of BeOS that can run BeOS binaries natively, and uses the same desktop because Be made Tracker open source before it imploded.
So, still alive in a form, famous and celebrated and covered in the mainstream media at the time.
Also:
* The subject of a high-profile lawsuit between Microsoft and Hitachi.
* The subject of an Apple acquisition struggle with NeXT over who would be the new MacOS.
* Indirectly the basis of the final version of PalmOS, the #1 handheld OS of the 1990s.
* Also the basis of YellowTab/Magnussoft Zeta.
* And of numerous remixes such as BeOS Max.
Only "forgotten" if you weren't paying attention.