Implement Option to Make Dataframe Unselectable!
If you're suggesting this for desktop/ArcMap, I wish you luck, there's not going to be any more ArcMap releases, 10.8.1 is it... https://support.esri.com/en/arcmap-esri-plan (presumably why there's no ArcMap ArcGIS Ideas page anymore).
For Pro, I agree that could be useful. Make layout elements selectable/unselectable just like you can for layers in a map.
A decent workaround and one that I now use exclusively (though it is a considerable change to workflow/muscle memory) is to make sure my elements are properly named and use the contents pane to select exactly which element/s I want to do something with instead of clicking around in the layout.
Luke is correct that no new features or functionality are being added to ArcMap. I realize that ArcGIS Pro was not around when this idea was submitted! It looks like this is available using Production Mapping in ArcMap: https://desktop.arcgis.com/en/arcmap/latest/extensions/production-mapping/locking-and-unlocking-elem...
And the cool thing is that this is built-in to ArcGIS Pro for everyone to use.
https://pro.arcgis.com/en/pro-app/latest/help/layouts/layout-contents-pane.htm
Here I have the map frame locked so that it cannot be selected in the layout while the other elements can be.
Just like when working with layers of a map, you can select multiple layout elements in the Contents pane and hold Ctrl +click the lock to toggle these on/off.
Hope this helps present-day workflows!