The Office team has deprecated the "Edit Offline" functionality for SharePoint documents synced to Outlook. Our user data shows that this feature is infrequently used, but we wanted to provide a way for users to provide their feedback on this change.
This change affects Office 365 subscribers only.
To share your opinion, leave us feedback on User Voice.
We recommend that you sync SharePoint libraries to your computer using the OneDrive sync app, which does let you edit offline and automatically saves your changes to SharePoint once you go back online.
Learn more: Sync files with the OneDrive sync app in Windows