Currently, the irc.wikimedia.org and RCStream feeds do not contain any information about change tags.
- change tags can be (and are) applied after the change is made
- feed lines are already too long sometimes
- preserving backwards compatibility with existing clients is important
the act of tagging would need to be sent to the feed as a separate line, in contrast to Recent Changes where tags are displayed alongside changes.
All feed clients that I am aware of apply line-based pattern matching and ignore unrecognized lines, so this should not break anything.
See related T20080, which requests that the AbuseFilter extension includes details of filter hits. Since change tags are a core feature, it would be cleaner to include this in core, and not have to re-implement it for any other extensions that start using change tags.
This is mostly a task for RCFeed as emitted from MediaWiki core. This is not a task for RCStream or EventBus, those interfaces will naturally contain this as part of the core RCFeed format.