I’m surprised WaniKani doesn’t have this feature yet, but I tend to slip when reaching for the enter button and accidently typing ]. Resulting in this…
They probably could, though it seems a little overboard
WK still uses an old version of WanaKana, as they probably don’t wish to risk upgrading (if it ain’t broke…).
Though they wouldn’t need to for this, like you suggested they can just add a regex check on the answer submission after conversion anyway (since they may as well do the same for english/meaning answers too).
For reference, these (plus english punctuation) could be used to throw the “yellow invalid input shakey shake”.
Unicode JA punctuation ranges (excluding ー [長音符] & 〜):
In fact, the double check script is causing this bug. I mentioned something similar to @rfindley a week ago and I just verified that ] shakes when double check is off.