Duolingo used to be a very effective gamified language learning system.

Then they decided they cared more about profits than providing a quality product.

Now they are best known for their dark patterns.

Yep, I noticed recently that on their official research portal they stopped publishing anything publicly in I think 2021. The last couple of articles before that were along these lines:

https://research.duolingo.com/papers/yancey.kdd20.pdf

I assume all the research after this point is too revealing to publish.