Instagram has been testing the ability to save posts as drafts since at least the end of July. With today's Instagram 9.4.0 update, the feature appears to have gone live for everyone.
To save a draft, simply tap the Back button while editing a photo to bring up the below message. To access your saved draft, tap the Camera button at the center of Instagram's bottom bar, and it will appear in the Gallery section above your pictures. This works exactly as you would expect post drafts to work.
Instagram has not announced any other features in the 9.4 update, but you can grab the update from the Play Store or the APKMirror link below.
The APK is signed by Instagram and upgrades your existing app. The cryptographic signature guarantees that the file is safe to install and was not tampered with in any way.