Block Topic Selection on Questions
You can block topic selection when a user asks a question in a community topic from a mobile device. For example, with blocking in place, a user who selects a topic and then asks a question cannot select a new topic in the ask dialog. Instead, the dialog shows the read-only current topic in the Posting to field.
Where: This change applies to Lightning communities accessed through Lightning Experience and Salesforce Classic in Essentials, Enterprise, Performance, Unlimited, and Developer editions.