Yes. I know the topic. As far as I know it has been fixed long ago.
I haven’t noticed a noticeable difference on my machine when I’ve looked into it with iotop.
I don’t run torrents on btrfs file system, it may be one reason why usage is not rising.
But I noticed several gigabytes of writing with the iostat program (before and after) the defrag command.
Choices, choices.