As reported by The Verge, Twitter started sending emails to the holders of accounts who had not signed in for six months or more:
Brandle®, Inc. is dedicated to providing a comprehensive system for companies to manage the properties of their brands, identities, and relationships across the web and on all major social networks including Facebook, Twitter, YouTube, Instagram, LinkedIn, VK, and more. It is our mission to be the trusted source for social media governance and web presence management and to provide functionality that helps every enterprise manage, secure, and protect their brands.