The recommendation of 5% free space comes from arrays in sizes of 100s of GB, not tens of TB. This is the same kind of issue that Linux root filesystems had -

The recommendation of 5% free space comes from arrays in sizes of 100s of GB, not tens of TB. This is the same kind of issue that Linux root filesystems had - a % based limitation made sense when disks were small, but now they don't make a lot of sense anymore when they restrict usage of hundreds of GB (which are not actually needed by the filesystem to operate).