Disabling languages in Drupal 7 was used as a kind of content staging solution. You could assign disabled languages to content *but* only if the translation module was enabled (not purely with node/locale modules). And the language was not yet visible on the language switcher block. So when time came to publish the language, you enabled it and it worked. Well, no.
Path aliases were not supported to be saved for disabled languages, menu items were still visible, you could not actually switch to this language for testing because the language negotiation system never supported disabled languages, etc. All-in-all it was a minimal implementation of a confusing feature only under special circumstances. We looked at solving this but in the patches, it just resulted in even more complication, so we decided we better remove this feature.
If you need content staging, you can unpublish your nodes in soon to be released languages (the switcher links will not appear for those either); apply content permissions or in more complex scenarios, just use a development/staging server with a staging to production workflow.
In Drupal 8 all languages should be considered enabled. Context specific limitations of the language list are on the table to improve user experience when assigning language to content for example, stay tuned.
