Your question has two parts. The first part, "How do you find questions to answer?" is a duplicate of How to find the right questions that I can answer?.
The second part of your question, "Why is the most obvious feature, namely to list unanswered questions with a specific tag, still not implemented? Is it because no-one actually needs it?" is a duplicate of Is there a way to get truly unanswered questions of a certain tag?
If you believe your question is different, then you have a few options:
- Re-write the question emphasizing the differences and why the existing questions don't help.
- Comment on the question explaining the differences
- Flag for moderator attention and request that it be re-evaluated in light of your changes and comments
- Re-post a new question with the differences emphasized
- Post on meta and request the community explain why the moderator took a particular action (which is what you did, so you're already on the right track)
But at the moment, your question is not differentiable from the existing questions.