A decoy wallet... for when someone gets your seed phrase? It's unlikely that someone will make their paraphrase long enough to prevent someone from brute forcing it. If you want a decoy seed phrase, why not change a few words and fix the checksum? At least then it's less obviously a decoy... "25 words" + low balance = decoy that you should run a dictionary attack on. 24 words with low balance is a wallet

Obviously you have make it long enough! 😅

So someone should be not enough educated to use a weak passphrase, but has the competency to recalculate a checksum? I repeat: it's an advanced feature, and like all advanced features it requires a precise application in relation to goals and circumstances. For sure, every UI that exposes the passphrase possibility must do it carefully, use the correct terns ("25th word" is just wrong) and point the user to a good informative resource.