field_info_field() deprecated, what's the equivalent?
According to the change record:

field_info_field($entity_type, $field_name) -> FieldStorageConfig::loadByName($entity_type, $field_name)

So you should use \Drupal\field\Entity\FieldStorageConfig::loadByName from now on.

It's probably best to use change records and issues on Drupal.org itself for docs at the moment, rather than 3rd party websites - I think they have slightly more chance of being right and kept up to date when something changes.