The listing for the Wizard: School of Onomancy lists Durable Magic as a 10th level feature. The article that Onomancy was published in does not show Durable Magic as a feature of Onomancy. Here is the link to that article for ease of access: https://dnd.wizards.com/articles/unearthed-arcana/cleric-druid-wizard
Durable Magic is the 10th level feature of the War Magic subclass for Wizard however. I believe the feature was likely accidentally pulled in when the Onomancy listing was created.