Before the most recent updates to ArcGIS Online, we had the ability to manually updated feature service JSON's to modify a domains split policy. It now seems this is no longer an option (see discussion here: Re: Splitting tool wipes values from AGOL feature ... - Esri Community).
It would be REALLY USEFUL if we had an option to update feature service Domains the same way we can with geodatabase Domains using the "Alter Domain" tool.
![LindsayRaabe_FPCWA_0-1656989042921.png LindsayRaabe_FPCWA_0-1656989042921.png](https://community.esri.com/t5/image/serverpage/image-id/45160i79FCAF244A8DBF4E/image-size/medium?v=v2&px=400)