From what I know there is no scientific proof of this practice because there has been no investigation to measure its effects. The closest to a test of dopamine fasting is to look at other practices that reduce dopamine, like meditation and mindfulness. These practices aim to escape the rat race of finding the next gratification.

Mediation and mindfulness are effective. They imply other techniques and learnings besides dopamine fasting. So we cannot know how much of their success comes from dopamine fasting, and how much from the other aspects. However their success is a potential indication that reducing dopamine significantly for a limited period of time can have significant benefits.

Besides this, it makes sense. Biologically if cellular structures can develop tolerance for something, the absences of that stimuli increases sensitivity.