Many people have asked for this functionality in AGOL, but it does not look like it has been posted as an Idea for ESRI management and development to see. As such, here is the idea.
This capability has been asked for now for at least 1.5 years.
For some context on this Idea, here are some threads from fellow users:
How to create a line break when concatenating text in ArcGIS online Arcade?
Arcade Text Constant for TextFormatting.NewLine is adding space instead of new line
https://community.esri.com/ideas/12902-add-support-for-arcgis-online-stacked-labels (in product plan)
Retrieving data ...