I know that Magento 2.0 is still under development and it doesn't have even an alpha release yet. Are we allowed to ask questions about it or we should limit to the released versions 1.x? I know that the issues can be reported on github, but what if we (I) have questions about "how to..." that are not bugs or feature requests?
In my opinion, questions about it shouldn't be allowed. Simply because, as you said, it doesn't even have an alpha release yet.
Features in it can change daily, stuff removed, stuff added. It is very likely that questions that would be asked about "How can this be done", "How is this implemented" etc... would have different answers once the full version is released. This will render those answers obsolete very soon, which wouldn't be helpful to the community. In fact, the answers can even be even harmful.
At some point we should start entertaining these questions, and in the StackExchange spirit I'd rather err on the side of facilitating content rather than not. Magento 2 is nigh and there is a wide-open tag field which can be used for people seeking information down the road.
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