Lots of mushy domains still have aspects of their foundations that are amenable to deliberate practice.
For software engineers, problem solving with math and leetcode style questions can be trained in a deliberate practice style, as well as (for example) learning the ins and outs of the most import parts of your language's standard library or the most used core unix utils, or how to construct various SQL queries.
Getting those out of the way means you can focus more on the fuzzy aspects.
So I'd say there's still a role for deliberate practice in broadening and deepening your foundations.
For software engineers, problem solving with math and leetcode style questions can be trained in a deliberate practice style, as well as (for example) learning the ins and outs of the most import parts of your language's standard library or the most used core unix utils, or how to construct various SQL queries.
Getting those out of the way means you can focus more on the fuzzy aspects.
So I'd say there's still a role for deliberate practice in broadening and deepening your foundations.
Dan Luu has written about similar things: https://danluu.com/p95-skill/