UPDATE: Hot on the heels of the last update, Microsoft released Azure AD Connect version 2.1.20 (6 days later) which apparently fixes a bug in the new sync feature described below. If you implemented the custom sync rule described earlier, you'll need to undo it and do it again. <facepalm>
Microsoft updated Azure AD Connect to version 18.104.22.168 today.
According to the release notes, "We added a new attribute 'employeeLeaveDateTime' for syncing to Azure AD. To learn more about how to use this attribute to manage your users' life cycles, please refer to this article".
Let's break this down.
First, the support article the release notes refer to is for Azure AD Connect cloud sync, not Azure AD Connect. At this time, syncing the employeeLeaveDateTime is still not supported in Azure AD Connect.
Since AAD cloud sync doesn't support Exchange hybrid at this time, AAD cloud sync is of no value for hybrid customers. However, the AD Connect team is working on adding Exchange hybrid support in the future.
The support table shows that syncing the employeeHireDate attribute is already supported in Azure AD Connect, so I expect that suspect the configuration is the same. I could not find a similar support article for Azure AD Connect.
Second, the employeeHireDate and EmployeeLeaveDateTime attributes only exist in Azure AD. The on-premises Active Directory schema is not extended to add these two attributes, so the recommendation is to use one of the existing extensionAttribute* attributes an AD to hold these values.
At this time, Microsoft recommends using the extensionAttribute1 attribute for employeeHireDate, but the documentation makes no mention of how to handle the EmployeeLeaveDateTime attribute. See How to synchronize attributes for Lifecycle workflows - Microsoft Entra | Microsoft Learn. I hope this documentation will be updated soon with the missing info and how to handle it if this attribute is already in use in your organization.
Last, if you want to update the employeeLeaveDateTime attribute directly in Azure AD using the Graph API, please see Set employeeLeaveDateTime - Microsoft Graph | Microsoft Learn.