Did the WordPress default database connection allow for the use of mysql_query() in years gone by? If so, at what point did this end? Am I assuming correctly that it has to do with the WordPress database object?

Background: Many years ago, I wrote a custom template for a WordPress client that involved importing an XML feed into the database and showing it on WordPress pages. I kept copious notes, but only on the trouble areas, and the database connection wasn't one of those. The client moved on, but is now back with an error in that old, old code. I can fix it, but my client is the curious sort and may want specifics on what changed.