A user asked in 2022 (https://forum.opnsense.org/index.php?topic=31219.0) what's correct way to add static arp entries to opnsense. They linked (https://forum.opnsense.org/index.php?topic=7273.0) to an older discussion where users were adjusting static_arp_pairs configuration directly in /etc/rc, but as he pointed out this would be overwritten by opnsense future updates.
I wanted to clarify that since around January 2024 opnsense provides Neighbors (https://docs.opnsense.org/manual/neighbors.html) menu that let's you insert such static permanent arp entries (applies after reboot).
I wanted to mention this as these posts seemed to score higher in internet search engines than the above documentation page, so I was hoping this post will get clumped with others and help out some future users.