We’ve been using similar techniques for awhile where I’m at with great success. We set aside every other friday for unstructured development work. Recommend this 100%.
From my experience, no software development process (kanban, scrum, whatever) can effectively capture all the things that need to happen to keep a product and a codebase healthy. Whenever I’ve been in an environment where all of the engineers' time is “spoken for”, it leads to bad outcomes.
Leaving some slack in the system that is dominated by individual human judgement is a great way to avoid this.