Is there a roadmap for RTC that shows what the minimum releases of Web Browsers ?

 Are you lookng for RTC 6.0.1 system requirement?  In the CLM 6.0 system req page, it says "For platform pressures (enhancements and more) please see the CLM Platform Plans and Pressures Dashboard."  I hope you can find the information.

I'm not aware of such a roadmap or any official policies, but when checking with previous system requirements, we can see that RTC supports the most recent two versions of Firefox ESR, and the most recent three versions of Chrome and IE (RTC 6.0 only supports IE10 and IE11, and leaves room for support of Spartan). As long as you can figure out what is the "current" version of the said browser when RTC is released, you should be able to figure out the "minimum" support version. Another way to find it out is by checking out the plan items on jazz.net, such as this one.
https://jazz.net/jazz/resource/itemName/com.ibm.team.workitem.WorkItem/336920