How to make Language Switcher Block work with unpublished nodes?
I was finally able to track this change down. The source lies within a security announcement which led to the i18n-7.x-1.11 update. In /sites/all/modules/i18n/i18n_node/i18n_node.module, l. 124 there's a new line reading

'access' =&gt; !$matches[2] ? $node_translation-&gt;status : NULL,


This line shall prevent unauthorized users from even seeing the path alias of nodes they do not have access to. However, there's no real check of permissions, this line just allows the access based on the node's status. So, even for User 1 (Superadmin) the link to the translated node is inactive if the translation is unpublished.

Whether this is only a temporary fix because of the SA or even supposed to be a permanent solution, I don't know. But that's the way it is right now and if you need to switch between unpublished translations often, you might want to comment that line.

There is already an issue at d.o for that.