ARB CBoot has been abandoned and given way to GRUB2. But it is not alone in abandoned operating systems. Many free OSs weren't known by the public and so when abandoned, they weren't missed. Going back some decades, there was Commodore. It was popular; the Amiga was well loved, but it was overtaken by Windows, as was Symbian by Nokia for its phones.
Symbian's UIQ also dive-bobed. Palm had quite a good run but was abandoned when everyone wanted access to the web and multimedia: it couldn't keep up. Other OSs that are very little used I haven't included if they aren't totally abandoned.
Computers have evolved greatly over the years and the same thing can be said for their operating systems. This brings to thought some of the oeprating systems that have been abandoned over the years. The AmigaOS for Commodore was the first computer operating system that was geared for video editing and making video games online. It came out in 1985. IBM's oS2 was a system that was around for years and was prevelent during the early years of the formation of the internet. MS-DOS was an operating system that most people that started using home computers became aware of. This computer operating system was influential in the early days of the internet and seen it being developed and updated over the course of 15 years until it stopped production and availability in the year 2000. There are plenty of other operating systems that been abandoned as well.