Ok, I just checked with some windows using friends (haven't touched a PC in 5 years) and the official eMule indeed does not seem to have a number of upload slots feature, which is kinda surprising because it is such a basic one, but many mods apparently do. But that does not change the fact that pretty much every other ed2k client has it, including mldonkey, which has it since at least the middle of 2001, which is when I started using it.
So, in summary, I'm not quite sure how you stand by your statement "aMule alone(save for Shareaza on Windows) has it available", when in fact it is "every client has it except for eMule".