We have a hard coded list of post meta fields and custom taxonomies that we put into the search index. That list is in class.jetpack-sync-module-search.php on wp.com. We need to:

• sync the list to jetpack and likely refactor it significantly
• use this list to filter what taxonomies are available in the UI
• add the list of post meta keys that can be filtered on (see Search: Index an allowed list of Post Meta for searching #8663)
• document and create examples of how to request adding post-meta/taxonomies to the list (probably on jetpack.com/support/search, but maybe also in the UI).