and uses btrfs send/receive to create backups.

I’m not familiar with that, but if it permits for faster identification of modified data since a given time than scanning a filesystem for modification times, which a filesystem could potentially do, that could also be a useful backup enabler, since now your scan-for-changes time doesn’t need to be linear in the number of files in the filesystem. If you don’t do that, your next best bet on Linux – and this way would be filesystem-agnostic – is gonna require something like having a daemon that runs and uses inotify to build some kind of on-disk index of modifications since the last backup, and a backup system that can understand that.

looks at btrfs-send(1) man page

Ah, yeah, it does do that. Well, the man page doesn’t say what time it runs in, but I assume that it’s better than linear in file count on the filesystem.