Bitcoin Core – What are rw_settings?

Here is the beginning of my log on Windows (Bitcoin Core v26): 2024-01-21T04:50:15Z Ignoring unknown rw_settings values ​​onion-prev 2024-01-21T04:50:15Z Bitcoin Core version v26.0.0 (release) build) 2024-01-21T04:50:15Z Qt 5.15. 10 (static), plugin=windows (static)

Do you know what “ignore unknown rw_settings value onion-prev” means?