Huh, right you are: the inline_data feature is from 3.8 (2013), but apparently tune2fs couldn’t enable it on a live filesystem for some time after that, and it’

Huh, right you are: the inline_data feature is from 3.8 (2013), but apparently tune2fs couldn’t enable it on a live filesystem for some time after that, and it’s still not on by default. (I remembered seeing the feature in the format description, but did not think it would actually be unused in most cases.)