> There are no (stable) alternatives. BTRFS certainly not, as it's "under heavy development"¹ (since... forever).
Note that they don't mean "it's unstable," just "there are significant improvements between versions." Most importantly:
> The filesystem disk format is stable; this means it is not expected to change unless there are very strong reasons to do so. If there is a format change, filesystems which implement the previous disk format will continue to be mountable and usable by newer kernels.
...and only _new features_ are expected to stabilise:
> As with all software, newly added features may need a few releases to stabilize.
So overall, at least as far as their own claims go, this is not "heavy development" as in "don't use."
Note that they don't mean "it's unstable," just "there are significant improvements between versions." Most importantly:
> The filesystem disk format is stable; this means it is not expected to change unless there are very strong reasons to do so. If there is a format change, filesystems which implement the previous disk format will continue to be mountable and usable by newer kernels.
...and only _new features_ are expected to stabilise:
> As with all software, newly added features may need a few releases to stabilize.
So overall, at least as far as their own claims go, this is not "heavy development" as in "don't use."