We've relocated the Import CSV button and all associated functionality into the Import history tab of the address book:
Clicking on the "Import history" tab will show a list of all previous CSV imports (if applicable) and enable the "Import new CSV" button.
Click the eye icon to the right of any previous CSV import to view a list of stops that are associated with that imported CSV:
From here, click "Add all to custom stops" to migrate the imported data from this CSV into the Custom stops tab of the address book for quick addition to routes:
Additional details on CSV import workflows, file format requirements, and working with imported custom stops can be found in the support article for this feature.