puppet 4.9 added code to parse and validate the hiera.yaml[1] file and will now throw an error if there are top level keys defined which do not exist in the :backends: array.
We see the following errors
On the client
Error: Could not retrieve catalog from remote server: Error 500 on SERVER: Server Error: Evaluation Error: Error while evaluating a Function Call, Lookup of key 'lookup_options' failed: The Lookup Configuration at '/etc/puppet/hiera.yaml' has wrong type, unrecognized key 'private' The Lookup Configuration at '/etc/puppet/hiera.yaml' has wrong type, unrecognized key 'secret' (file: /etc/puppet/manifests/realm.pp, line: 24, column: 14) on node jbond-buster.puppet.eqiad.wmflabs
on the puppet master
Jul 11 13:00:08 jbond-pmaster-buster puppet-master[29288]: The Lookup Configuration at '/etc/puppet/hiera.yaml' has wrong type, unrecognized key 'secret' (file: /etc/puppet/manifests/realm.pp, line: 24, column: 14) on node jbond-buster.puppet.eqiad.wmflabs
[1]https://github.com/puppetlabs/puppet/blob/4.9.0/lib/puppet/pops/lookup/hiera_config.rb#L301-L307