I started on this and realized we need to implement the adapter so that the deprecation warning isn't always triggered when using classifier terminus, etc, so putting this in needs information.
Calling Puppet::Indirection::Rest.{find,search,save,destroy,head} is deprecated as part of an effort to separate the indirector and http requests. If a custom terminus extends the abstract rest terminus, then it should implement the networking request itself or call puppet's http client like the builtin rest termini do, such as Puppet::Indirector::Node::Rest.