ISO 9001 and complaints handling

Answer:
In ISO 9001:2015 you can find mention to complaints in clauses 8.2.1 c); 9.1.2 (in a note) and 10.2.1. There is no reference to a mandatory procedure, but an organization with a quality management system:

must plan how to communicate with customers when handling complaints;
can include complaints as an input to the customer satisfaction assessment methodology; and
should evaluate the need for corrective action after receiving a complaint

The following material will provide you more information:
ISO 9001 – Effective complaints management in a QMS - https://advisera.com/9001academy/blog/2014/09/16/effective-complaints-management-qms/
Free online ISO 9001:2015 Foundations Course - https://advisera.com/training/iso-9001-foundations-course/
Book - Discover ISO 9001:2015 Through Practical Examples - https://advisera.com/books/discover-iso-9001-2015-through-practical-examples/