The BitTorrentSpecification says
The piece length specifies the nominal piece size, and is usually a power of 2.
The https://www.bittorrent.org/beps/bep_0052.html says:
The number of bytes that each logical piece in the peer protocol refers to. I.e. it sets the granularity of piece, request, bitfield and have messages. It must be a power of two and at least 16KiB.
So it should not be is_divisible_by_16_kib