According to Kent on Reddit, bcachefs technically had online defrag since it was just bcache -- via the copying garbage collector for reclaiming space. So bcachefs will simply inherit that feature by design, which is great.
Apparently copygc is off right now because reasons, though (I'm going to assume it's almost certainly the related extent/compression issue that's holding this up from being enabled, which you can see referenced on the home page, at the bottom).
I should have been more clear - copygc is off by default in upstream bcache, it's on in bcachefs (and required, in order to guarantee a capacity when doing random writes)
Apparently copygc is off right now because reasons, though (I'm going to assume it's almost certainly the related extent/compression issue that's holding this up from being enabled, which you can see referenced on the home page, at the bottom).