On some topics, I want to limit my queries to a specific version of either a programmimg language, library, or framework. This happens most often with fast-moving open source projects (angular, ember, rails etc...) and would only be applicable for certain tags (maybe moderator rights to select). I know the common pattern is to have a separate tag like
ruby-on-rails-3, but this is a bit ugly since it require massive duplication and more importantly missed when somebody tags something ruby-on-rails-3 and you only search ruby-on-rails. I don't think this is a feature that would be applicable to most tags, but for certain fast moving apis, it would provide more information than is currently available and reduce duplication. I'm thinking this would be something that could co-exist with current tag system but use user-participation to make work better. I think as SO gets older this becomes more and more of an issue (ie looking at 5yo Rails questions can be interesting... and this problem will just get worse). This is probably relevant for like 10% of topics here.
In terms of how this could be implemented, the asker would select
ruby-on-rails and her version for the question (lets say 3.2). Somebody (either an answerer or reader of the question) could then either expand the range of applicability of the question (say for rails it is applicable from 2.2 to current). This way the scope of the applicability of the question and answer could be specified very explicitly and outdated answers could be made explicit.
I think this would also help the question asker by reminding them to specify which version of the framework they are using. Also, the more I use the search features, the more I would like to use operators for only Ruby on Rails >= 3.2. It seems managing this at the actual tag name is ugly. This could be done over time and could coexist with current tags.
(I have the feeling this has been asked / suggested so feel free to point to the discussion but I couldn't find it. I see this and Jeff's answer here as closest to addressing: When is it appropriate to have minor version numbers for tags?. The gist of his argument is that version specific tags should only be used when the topic in question is specific to the question.)