The crown system is a current feature of Duolingo, replacing both the fluency badge and the skill deterioration mechanic. Unveiled for all users throughout the month of April in 2018, it provides a way for experienced users to get more complex lessons and a more enriching experience through a level system.
Before beginning a skill its crown and circle are in gray. When one finishes learning all of the lessons in a skill, the skill's circle becomes fully colored and gains a Crown Level of 1. If this skill is practiced again, it will begin to accumulate points until it reaches Crown Level 2. There are five crown levels, and the amount of times required to level a skill to the next crown level is the same as the original number of lessons in the skill:
Once a skill has been leveled to Crown Level 5, it turns a golden color and is considered mastered. A mastered skill can be practiced. You can also test out (making no more than three mistakes) to advance to the next level.
As of 24 October 2019, spaced repetition has been reintroduced to Duolingo. When a skill is at Crown Level 5, it may "break", requiring a review to put it back together again.
One of Duolingo's problems prior to this feature was the lack of engagement for those who had completed all of the skills of a language tree and wanted more complexity in terms of content. So an A/B test of new Android users began in late 2017. Having been successful in enriching the language-learning experience, it was rolled out to all existing users on all devices in the first week of April 2018.
Its arrival was met with mixed reviews, with some users saying it deepened their experience with Duolingo and others claiming it was unhelpful and/or confusing. Some bemoaned the fact that spaced repetition and word lists are unavailable, but the Duolingo staff assured users that these features would be resumed at a later date.