apply_filters( 'relevanssi_remove_stopwords_in_titles', boolean $remove )
This filter controls whether Relevanssi removes stopwords from titles in tokenizing or not.
Parameters
$remove
(boolean) If true, Relevanssi removes stopwords from the titles. If false, Relevanssi doesn’t remove them. The default value is true.
More information
When Relevanssi indexes post titles, the title first goes through filters. Relevanssi applies all the_title filters first, then comes relevanssi_post_title_before_tokenize, and then Relevanssi tokenizes the title.
In tokenizing, the default setting is to remove the stopwords in the title. If you want to keep stopwords in the titles for some reason, you can have this filter hook return false:
add_filter( 'relevanssi_remove_stopwords_in_titles', '__return_false' );
Now when you index posts, stopwords are removed everywhere else, but not in the post titles.
With Relevanssi Premium, it’s better to use content stopwords.