Currently, we have two ways to connect a Confluence calendar and a mailing app like Outlook or Gmail. The steps for them are below

https://support.atlassian.com/confluence-cloud/docs/subscribe-to-team-calendars-from-microsoft-outlook/

However, this would require the Disable Private URLs to be unchecked which can make the calendar accessible to anyone who subscribes to it. This feature request is to have a more secure way of connection.