systemd-cryptsetup: Encountered unknown /etc/crypttab option '_netdev', ignoring.

Issue

• When _netdev option is added to /etc/crypttab, this log starts appearing:

  # systemctl status systemd-cryptsetup@srv.service -l
  ● systemd-cryptsetup@srv.service - Cryptography Setup for srv
     Loaded: loaded (/etc/crypttab; bad; vendor preset: disabled)
     Active: active (exited) since Pi 2020-10-30 16:20:27 CET; 56min ago
       Docs: man:crypttab(5)
             man:systemd-cryptsetup-generator(8)
             man:systemd-cryptsetup@.service(8)
   Main PID: 682 (code=exited, status=0/SUCCESS)
  
  okt 30 16:20:21 server systemd[1]: Starting Cryptography Setup for srv...
  okt 30 16:20:21 server systemd-cryptsetup[682]: Encountered unknown /etc/crypttab option '_netdev', ignoring.
  okt 30 16:20:24 server systemd-cryptsetup[682]: Set cipher aes, mode xts-plain64, key size 256 bits for device /dev/vgsys/srv_encrypt.
  okt 30 16:20:27 server systemd[1]: Started Cryptography Setup for srv.
  

This option is expected to be supported - at least according to man crypttab.