What are the changes to the Hosted Feature Layer Refresh Interval at the September 2020 ArcGIS Online Update?

The shortest possible refresh interval that can be set on a hosted feature layer changed from 6 seconds to 30 seconds at the September 2020 ArcGIS Online Update.

ArcGIS Online hosted feature layers are used to manage and share up-to-date, (near real-time) data with your intended audience. Although hosted feature layers support sub-minute data updates, hosted feature layers were not architected to visualize live, sub-second data updates at scale. For this reason, the ability to set the refresh interval in web maps to less than 30 seconds has been removed as of the September 2020 ArcGIS Online update. 

Esri offers several products that provide real-time visualization support, such as ArcGIS Velocity and GeoEvent Server. If there is a need to display data at an interval less than 30 seconds Esri encourages users to investigate these alternatives.