A lot of apps are nagware, but I’ve never seen any as blatant and forceful as Duolingo.
They diligently A/B test their notifications, constantly looking for the variations that’ll show a higher click rate.
They’ll hit you with “[Your friend’s name] will hate you forever if you don’t do Duolingo right now!” if you start slipping.
I’ve dropped it after a year or so when I realized that I wasn’t really learning any Turkish, but I was caught in some sort of corporate-designed psychological trap.