Email-to-case is a channel that meets the requirements of logging the case thread and storing attachments to the case record. Email-to-case is a feature that allows you to automatically create cases from incoming emails sent to your company's support addresses. Email-to-case preserves email formatting and supports HTML emails with embedded images. Email-to-case also captures email attachments and adds them to the case record. Email-to-case tracks the entire email conversation between the customer and the agent, and logs it as a case thread on the case feed or case comments. Verified References: Service Cloud Consultant Certification Guide & Tips, Email-to-Case Overview

Explanation
Omni-Channel Supervisor is a solution that can enable supervisors to respond to flags and provide responses that only the agent can view. Omni-Channel Supervisor is a tool that allows supervisors to monitor and manage the work and performance of agents in real time. Omni-Channel Supervisor can help supervisors respond to flags raised by agents who encounter issues and provide private coaching messages that only the agent can see. Verified References: :
https://help.salesforce.com/s/articleView?id=sf.omnichannel_supervisor_overview.htm&type=5 :
https://help.salesforce.com/s/articleView?id=sf.omnichannel_supervisor_flags.htm&type=5 :
https://help.salesforce.com/s/articleView?id=sf.omnichannel_supervisor_messages.htm&type=5