Windows 9x/NT 4
Why doesn’t Legacy Update work on Windows 95, 98, Me, and NT 4?
Legacy Update works because we’re lucky enough that the Windows Update v6 protocol is pretty much the same today as it was when it was first released. As such, it simply proxies all protocol traffic to Microsoft’s official Windows Update servers. It seems unlikely that Microsoft will completely discontinue Windows Update for 2000/XP.
Windows 95, 98, Me, and NT 4 used Windows Update v3, with Windows Update v4 later coming to Windows 98, Me, and NT 4, in addition to the initial releases of Windows 2000 and XP. (v5 existed for a brief period before v6, but they are almost identical.) Both versions are now discontinued, making a reimplementation of them more difficult. In the case of v3, there are many catalog files and executables that need to be appropriately recovered from archives of the Windows Update service. In the case of v4, in addition to update executables needing to be recovered, a database also needs to be rebuilt.
I’m looking into making use of Windows Update v4 to provide a happy medium between Windows Update v3 and v6. It’s a bit more robust than v3 in a few ways, and isn’t an overly complicated protocol like v6. While v4 never officially supported Windows 95, I believe it should be possible to make it work there anyway (it will require Internet Explorer 5 to be installed first).
You can try the unrelated Windows Update Restored project, based on a revival of the Windows Update v3 website.