Hi Hank Pritchard,
From the documentation on setting a Feature Layer's extent, it does not constrain edits: Item details—ArcGIS Online Help | Documentation. I tested this quickly to confirm.
Creating offline areas in Collector does enable something like this: Prepare for offline data collection—Collector for ArcGIS | Documentation
You may also be able to configure some attribute auto-population that would flag features outside the study area. You could look into the Smart Editor widget—Web AppBuilder for ArcGIS | Documentation or https://community.esri.com/community/gis/web-gis/arcgisonline/blog/2018/12/10/overlapping-features-i...
I would also consider creating an https://community.esri.com/community/arcgis-ideas for this functionality, I did a quick search and am not seeing any existing ones on it.
I hope that helps,
I know it's been several months since you addressed this question but I wanted to reach out to say thanks for taking the time to look into this for me.
I apologize for taking so long to respond but this particular map got pushed to a back burner for me and I forgot to follow up. I will, however, consider an ArcGIS Idea for this functionality.