Static ARP Entries were used to bind a MAC address to a specific IP address. On the latest RutOS 7, you can find Static IP leases section (Web Ui -> Network -> Interfaces -> LAN page). Static IP leases are used to reserve specific IP addresses for specific devices by binding them to their MAC address. This is useful when you have a stationary device connected to your network that you need to reach frequently, e.g., printer, IP phone, etc.
Note to make the Static Lease section visible, set interface protocol to Static.