It did, but as far as I know it's effective on fresh installs only. It would be too risky to migrate automatically to Btrfs from ext4 during the 32->33 upgrade

It did, but as far as I know it's effective on fresh installs only. It would be too risky to migrate automatically to Btrfs from ext4 during the 32->33 upgrade. I suspect a lot of users upgraded to 33 from 32 without seeing any problem with sticking with ext4 (including me). As a result, a lot of users are still using ext4.