Over the weekend, the Opera Mini browser was found to be removed from the Windows Phone Store, with no official word from the developer. Opera has since clarified the matter, stating that an "unexpected hiccup" caused the issue, and that it is currently working on reinstating a beta version of the browser to the store shortly.
Unexpected hiccup in WindowsPhone store. Working on getting opera mini (beta version...for now) back to you all :) #weloveWP /Ruth— Opera (@opera) February 2, 2015
There is no mention as to why the app was pulled from the store in the first place, but the wording of the tweet indicates that a finished (non-beta) version of the browser is in the pipeline for Windows Phone. We'll let you know as soon as it is available again.
Source: Twitter (Opera)
Thanks for the tip Laura!