We need a way to more specifically and reliably filter Wikibase recent changes in the client. We can't just rely on rc_type == RC_EXTERNAL, as something else might use that. rc_source seems to be the right choice, Wikibase puts "wb" there.
Note that https://www.mediawiki.org/wiki/Manual:Recentchanges_table#rc_source sais that rc_source "replaces" rc_type (though the docs for rc_type don't say anything about it being deprecated).