A ticket was shared with our account and was unshared after. Can I send public comments to the requester on an unshared ticket?
No. The dummy user created for ticket sharing does not have the same email address as the user. If you want to communicate directly to the requester, update the requester with a profile containing an email address of the intended recipient.
For more information about ticket sharing, see the article: Sharing tickets with other Zendesk Support accounts.