I was using Viber on my old phone. I stopped using that SIM card/number and stopped using that phone. I forgot to deactivate my Viber account. When my old SIM card carrier sells my old number to a new person, does that person access my Viber contacts and people I was talking to?
Assuming the platform works similar to WhatsApp or Signal Private Messenger. Viber should only use your phone number as a unique identifier, and no content (contacts or messages) should be stored on the server side.
This seems to be confirmed by Viber such that message backups are done through a cloud service; Google Drive or iCloud. Also, Viber uses end-to-end encryption, so their servers cannot see the content of messages, only metadata.
Consequently, if someone does receive your old phone number, no messages can be seen (as no content is stored by Viber), and the same is applicable to contacts.