Some components are improved continuously. So, "since" mark does not guaranty that the component will look and work like in the demo if you use older versions of RichFaces.
I understand there is no guarantee, but a since of 3.3.0 would tell me that I can't use it in 3.1.6 for instance. If there was an added attribute, then the attribute could be marked since 3.3.0 while the widget itself might be marked since 3.1.0 (all attributes not marked are assumed to be valid since the widget was created).
I believe this is how the normal Sun java classes are documented and I think that it would help a lot for people like me who can't (for one reason or another) always use the latest version.
I don't necessarily think bugs should be marked (if there was a bug fixed, then there will be a JIRA issue logged against it and we can check that if we think the behavior we are seeing is not correct), but I think that a basic implementation "since" field would be very useful.