I really wonder how this question even came into being. Have you come across any instruction to download Jetty that is missing to provide a version to download?

That would be important to know so that we can fix the problem.

I don't think that happened. There is a Rational Team Concert Extensions Workshop out there that we suggest users should perform. There is also a very old wiki entry that is really hard to find: https://jazz.net/wiki/bin/view/Main/RtcSdk30

As far as I am aware there is no and has never been any instruction (or even need) to download Jetty for RTC SDK development, simply because Jetty is contained in the SDK and its download.

I would suggest you basically follow the Rational Team Concert Extensions Workshop - mind the version you want and if you have questions while following it, ask these here. 

I would actually suggest to start here https://rsjazz.wordpress.com/2015/09/30/learning-to-fly-getting-started-with-the-rtc-java-apis/ if you want some additional information that I could not fit into the workshop.