I personally think it would be quite useful. Knowing when to expect to see words in kana alone would be helpful in real usage and it would still serve the purpose of using said Kanji to demonstrate.
I like this idea, maybe a little badge under the word that shows up in the lesson and detailed info pages. I think it’d be especially useful for people who start WK really early. I know we get a lot of people who type 今日は instead of こんにちは and stuff like that on the forums. It’s an easy mistake to make that WK could help users avoid with a little extra QOL feature.
There are some plants and animals that comes to mind. Those are nearly always marked on Jisho as “usually written in kana”. Along with some really common phrases, like what you mentioned @phyro. So, yeah, I agree. This could be helpful information for newcomers to Japanese.
Though, I guess, it’s a contextual thing: what type of text dictates which words gets kana or not, so you’ll still have to learn how to gauge literary context to know what makes more sense: kana or kanji? I have defo encountered 有り難う【ありがとう】so it really is about what type of book or manga you’re reading or what type of text you’re writing!
last comment on this thread was 6 or 7 months ago, but this would also be helpful. I just came across “subete” for “all” in vocabulary, as well as the verb “aru” for "to exist (inanimate objects), both using kanji even though i’ve only ever seen them in only hiragana in my years prior of off and on study (mostly with Japanese From Zero before starting wanikani) and following a bunch of JP music groups on SNS. Until this point, (lesson 6) I didnt see any others that obviously looked off.