See Data backup overview. Since changelog is talking of Android 6 compatibility, it is referring to Android auto backup (see table).
Enabled by default. Apps can opt out by disabling backups.
Contextually, it implies Textra had opted out of back up earlier and now has it enabled.
There is no option for the user to choose which apps can be backed up or not. ...
That account was set up 7 years ago and on that phone (S7) 4 years ago. In a different country. With different phone number.
It was working just fine until now, and only just now I realized that the phone numbers didn't match.
After changing phone numbers on the old phone WhatsApp, the new phone WhatsApp found the backup and successfully restored it.
It is possible to recover Android settings through bmgr.
List your sets using bmgr list sets
36636c7730288e33 : phone
3c559c36e65a9b49 : tablet
Now you can restore com.android.proviers.settings with the set you want:
adb shell bmgr restore 36636c7730288e33 com.android.proviers.settings
If you have Google Drive on your Android device then open it and go to Backups. Tap on the device's name you're interested in to see what's been backed up and when - App data, Call history, Device settings and SMS. Tap on App data to see the when each of your apps was last backed up.