I really do enjoy Wanikani, a great app for memorising kanji and it has helped me finally take action to uninstall or hide the many other Japanese apps I have acquired over the years. This is definitely the one for me.
I wondered if a Katakana words one would be a possibility? I know it is a moving target with new words appearing in society due to world events and online words as well, but wondered if this has been considered or ask for previously?
I guess it could be Katakana words and maybe even slang words as well to beef up the content.
Anyway just an idea and wondered whether it would get enough support for it to go ahead if it hasn’t been considered before.
I’m not sure about an app with just katakana words, but I know that Torii teaches you a 10k vocab deck, which has a large chunk of katakana words in it. So you learn katakana words and you’ll improve your grammar.
It also has a setting to remove all vocab presented in Wanikani, so that you won’t learn the same word twice.
I’m pretty sure Kitsun.io has multiple decks that are specifically katakana focused, but it is a paid service, so keep that in mind.