Exclude node from search API results based on values in existing fields
Given no alternative answers, I solved this problem by


installing the Search API Exclude Entity module
adding a field of type Search API Exclude Entity named search_exclude to the content node
adding the following to my custom rsc module

function rsc_node_presave(EntityInterface $node) {
  switch ($node-&gt;getType()) {
    case 'content':
      $node-&gt;field_search_exclude-&gt;value = &lt;conditions to exclude node&gt;;
      break;
  }
}

going to /admin/config/search/search-api/index/default_solr_index/processors
checking the Search API Exclude Entity box
selecting the Search API Exclude Entity tab at the bottom of the page
checking the field_search_exclude (content) box
reindexing the content


Seems to be working well. When unverified nodes are saved, the search_exclude field is set and that keeps the nodes out of the search results.