The perceived significance of the reward is irrelevant and doesn't affect whether its degradation is "enshittification" or not. It's not excluded from that categorization because it's considered a "trivial" feature. In fact, I'd argue it's specifically a good example of enshittification because it's one of the very few things that's actually just "a nice thing" the developers added, that was previously a nice friendly thing to grant when someone gains 1000xp in a month. A cool badge. Fun. They didn't have to have this nice little thing, but they did. That's cool, right?

If you use the app regularly (as you "should" if you're learning a language), you will absolutely certainly get the badge. Well, that is, until recently, because some of the quests can be difficult to accomplish. No longer is it just a nice consistent monthly thumbs-up in the form of a fun little badge, it's now a source of uncertainty and perhaps even a source of anxiety for people who are driven by "collecting" (e.g. "achievement hunters"). Sure sounds like enshittification to me, regardless of how big a deal it is.