The problem is much more severe on ZFS and other CoW filesystems, hence the common warning. Filling an ext4 filesystem up to 80%+ and then removing files might

The problem is much more severe on ZFS and other CoW filesystems, hence the common warning. Filling an ext4 filesystem up to 80%+ and then removing files might increase fragmentation a bit but it'll not have that much effect on performance. On ZFS, filling the fileystem up once can kill performance permanently on that pool.