Hello,
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.
Best regards.