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.

I wouldn’t call them effective as much as motivating. I think for people who would not be motivated otherwise, this methodology is fine actually, as the alternative is probably nothing. However if you are motivated, almost any other method is more effective then DuoLingo (or alternatives), including more effective then the old DuoLingo with the forums and everything.