chado_db_select() is deprecated in favour of the Tripal DBX Query API
The chado_db_select() extended the db_* procedural functions of the Database API (since deprecated) to support Chado. The Drupal versions of these methods were deprecated in Drupal 8.0.x and removed before Drupal 9.0.0. As mentioned in the linked Drupal Change record these methods were replaced by the modern object-oriented Drupal Database API. As such, chado_db_select() is replaced by Tripal DBX which is Tripal’s extension of the Drupal Database API.
chado_db_select()
Before:
$query = chado_db_select('feature', 'f');
After:
$query = \Drupal(tripal_chado.database)->select('1:feature', 'f');