Comment by Macha
I do think that Wanikani and Bunpro are kind of in a catch-22 on this compared to Anki. They've built their gamification features and UI on the idea that cards have specific buckets that they're in and something like FSRS is a lot more varied than that. Especially Wanikani, which has a system of unlocking more items based on your current items reaching a specific stage.
So I haven't integrated FSRS, and of these two, I've only used Wanikani, and have been playing with a reimplemention for Chinese hanzi: https://hanzi.bpev.me
But seeing how it's implemented, I think they could totally integrate something like FSRS to at least just replace their scheduling (how long until an item is next shown). The unlocking system can be implemented as a separate gatekeeping mechanism, and the buckets can be coded for certain step thresholds (instead of wanikani's "stage").
Basically, this is their entire srs system: https://docs.api.wanikani.com/20170710/#spaced-repetition-sy...