YAML can still be loaded from XML response bodies, it just doesn't parse response bodies that are pure YAML anymore. Something like <tag type="yaml">yaml here</tag>.
It's like ActiveRecord, where it comes with an adapter for several backends, but in order to use a backend you need to install a library gem which is not a dependency of ActiveRecord.
Oh hmm, I guess that's a good thing to point out. Yes, the multi_xml gem has not been upgraded, and the listed dependencies for httparty and mailchimp haven't changed either.