The introduction of the "Masking" option under Feature Layer > Drawing has been a major improvement for creating custom cartography. It however, falls short by not having the ability to accomplish the inverse at a layer level. There needs to be a "Clipping" layer option that functions exactly the same as Masking with the opposite results.
The "Clip Layer" option (still hidden) under Map Properties is very limited and doesn't give you the same flexibility needed. With it, you can only define 1 outline to use for the entire map which doesn't provide the same layer by layer customization found using the masking option. The purposed functionality would allow for the on-the-fly combination of multiple boundaries to form unique clipping areas. Plus, it would enable the application of different clipping boundaries to different layers in the same map. One could even apply both masking and clipping to a single layer to highlight certain areas without any extra geoprocessing. The options are endless from a symbology and labeling perspective if you have this flexibility.
There clearly are many workarounds to accomplish this, many of which require creating new features or manipulating your current data. But splitting features, adding additional attributes, creating snapshot copies of dynamic data, or manipulating data in general for cartographic reasons is not always the best approach from a data management perspective. Regardless, none of these approaches compare to the simplicity of clicking a check box that has no impact on the underlying data.