I found the oldest setup .exe application of Yahoo Messenger 5.5 (in this case it appears as build 1214, I think) Here it is: https://web.archive.org/web/20020805232500if_/http://www.gronik.cistron.nl:80/download/ymsgr_beta.exe
Pssst. It’s on OldVersion, too. 
What would be impressive, though, is if someone could get the installer for the original Yahoo! Pager. Now that’s a feat.
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the oldest version i found was 3.0