Why did you remove the Dictionary/Vocabulary tab?
There used to be a tab with the list of all words you already know. Now I can't find it.
It was removed as a part of the site's redesign which was rolled out to all users in January. Users who were part of the redesign's AB test lost it a couple months earlier. Duolingo is currently testing a replacement that is in many ways an improvement over the old version. If you don't have a "words" link on your menu bar, you're not a part of the test group.
As Alexis says, you're out of luck. Selections are entirely random. There are currently over 30 different AB tests being conducted, and most users are a member of only a few of them. It's worth noting that it's not, strictly speaking, beta testing (though there's an element of bug hunting/squashing involved). It's AB testing which tests the performance of those with the new feature against a control group (those of us without it) to gauge whether the feature actually benefits learners. If they're not satisfied with the results, they might withdraw the test to redevelop the feature or scrap it entirely.
The thing many people don't 'get' is that just because a feature is popular, or something users think they need, does not necessarily mean it actually enables learning. It is possible that people without 'words/vocab' may learn more because they use other resources that are better than Duo's.
I agree with you in general about the A/B testing. However, I still can't understand when and how making the words available would be a BAD thing for learning a new language. If I have to keep going to memrise and other websites to study my vocabulary, I'll just stay there. From a retention standpoint, Duo should be very nervous about their customers being told to go to another free website to study. It really is an awful business model if they ever want to turn a profit. I would think that by now they should know this is a feature that people want in some form or fashion. There is a balancing act of testing a feature's utility with supplying your customers with a feature that they want and believe they need. Their testing may prove that I don't need it, however, then it's up to them to convince me that this feature isn't necessary. Right now, all you get is silence from them, which bothers me because I would be much more patient and understanding if there was information about what they're testing and how the results are going.
I absolutely share your frustrations, having a list of words you're supposed to know is absolutely vital to learning a new language, and I just can't understand why would they remove it, even if it was half broken. This way I just spend more and more time on other web pages that have it.
So, since I have the 'words' thing at the top, that means I'm a part of the test? Not sure if I understood completely.
Yes, if you currently have the words page, you're a member of that test group.