Tagging on the Stack Exchange is an "emergent folksonomy," which means we let tags grow as the community sees a need and we don't curate tags until an actual problem crops up--then we fix that particular problem.
I'm not seeing any problem with rage yet, so let's wait and see how it does.
There is presently one question tagged rage: unsurprising as it is a new tag. Searching for "rage question", there are 113 questions across a multitude of systems that could benefit from it. There is a smaller number of hits from sneak-attack, another class feature, but all of these are associated with dnd or pathfinder. If sneak-attack merits a tag, rage certainly does.