Another theory is that Windows 99 was a rumored or conceptual version of Windows that never made it to production. In the late 1990s, Microsoft was working on several different projects, including a potential successor to Windows 98 that would eventually become Windows ME. It's possible that some internal documentation or conceptual designs referred to a "Windows 99" project, which was later abandoned or rebranded.
Between 1997 and 1999, Microsoft distributed beta builds of what was then called "NT 5.0" (later Windows 2000). Several of these beta builds—specifically through Build 1906 —contain boot screens and splash art that are wildly different from the final release. windows 99 iso