how to keep eth link down across reboots ?

[lejeczek <peljasz@xxxxxxxxxxx>]

    Hi guys.
      
      Perhaps not strictly _systemd_ question but community here surely
      is capable - a matter of me being lucky - how would you keep an
      Ethernet link/port powered down?
      I was thinking I'll try first _udev_ rules - given other
      tools/managers are told to stay away from the link/port
      Is there a better, best way to put such link/port down & keep
      it that way - naturally, please steer clear of "unplug the cable"
      type of ideas.
      
      many thanks, L.