How would you define deliberate practice, then? I would agree that deliberate practice may be hard to define across all skills, but it could be defined for a specific skill.

For example in programing, I would consider problem sets and side projects with functions and goals that are aligned but still outside of one's normal scope of work to be deliberate practice. Working on Project Euler problem sets could be considered deliberate practice. They are problems that can be done in most any programming language, are known to be great for learning a new one, but generally fall outside the usual work of handling data, transforming content, etc.

I would consider Project Euler to be akin to practicing scales on a musical instrument: playing scales is not necessarily music in a "having popular appeal" sense, but it does enable new skills and depth of knowledge of the instrument.