Do you have any plan to make this option available? It is often difficult for me to find content that has the right amount of unknown words; this would make the process easier and shorter.
We already show the % of new words, which is essentially the same thing. The sorting by this % caused the site to slow down, but we will re-introduce this ability in some form in the future.
I should add that if you are past the intermediate stage you should consider importing lists of words, going through them to find any words that you do not know, and then Updating. This will make the % new words indicator more accurate sooner.
I did that with French! I’m already at intermediate stage from my school days, but lingQ doesn’t know that. So when I started using lingQ for French I imported the most frequent 2000 words in the French language and marked them all as “known”. It makes my unknown words statistics much more meaningful.