The Attribute Assistant is a great tool. I would be very helpful If it was available for people using a collector app through ArcGIS Online or a custom Server app to have this available. It would save a lot of work/time in post processing of field collected data.
Attribute Assistant-like functionality is available through web services published by reference (referencing a registered data source) to ArcGIS Enterprise. See Share datasets with attribute rules—Geodatabases | ArcGIS Desktop .
If what you are looking for is support for Attribute Assistant-like functionality using ArcGIS Online, please go to Support for Attribute rules in ArcGIS Online to up vote - and it would be helpful to that development team if you would comment with your requirements such as what method/capability do you need for your work?