Newbie

It should be a simple humanstxt.schema.yml file defining the content and display_link settings schema.

$item->setExtraData('highlighted_fields', $item_fields);

$item_fields has to be an array keyed by the Search API field id / machine name and and array of highlighted HTML as value.

There's a known bug in SOLR 4.x that prevents More like this (MLT) queries from working when a numeric field is used in the comparisons. This was resolved in the the 7.x version of the module by adding some prevention code: #2004596: MLT not working with Solr 4.x and was ported over to the 8.x version. However, this code doesn't account for multivalue fields, which I believe have an "m" in them?

Like the database backend the solr backend should deliver highlighted excerpts, independent from the configuration if solr returns all fields or not.