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.
Taking the Project Euler example, consistently doing the exercises, if they go in difficulty for each done, is deliberate practise in (probably) algorithm design, but does nothing for architecture. But redoing the same exercises over and over, always improving the design of it in each iteration (examples: reduce number of lines, apply DDD or hexagonal, etc, though the solutions in Project Euler probably not the best for this kind of practise) improves architecture skills.
The idea of deliberate practise is to always be improving in the tasks you are doing, usually by taking more and more difficult tasks in progression, but also analysing the past actions to identify where to improve. For example, I always liked to play tennis (and have many years of competitive table tennis behind me). Before I started working with a coach, my practises were mostly playing the game, maybe having a few swings against a wall when I didn't have a partner. When I started working with a coach, we practises my service over and over and over again, changing targets in the field, the kind of spin, and analysing most of the services to see what went right and wrong with my service. I can only say my skills improved at more than 10x the speed than when I was just having matches with other people.
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.