Where did Wikipedia Pro app disappear to?
If an app you've been using isn't offered on the Play Store for some reason (other than being removed for legal reasons, or if it's malware), you can usually still find and install it from your install history.

From the web version of the Play Store, click on Apps, then click on My Apps on the left which should take you to https://play.google.com/apps which should show a list of all of the apps you've ever installed, including ones which have been discontinued, and you can click through to install them:



Alternatively you can do the same thing on your phone, open the Play Store app, go to My Apps, and then change from the "Installed" tab to the "All" tab and scroll down until you find your app (I think they're listed in the order that you first installed them):



(I'd advise using the web page, as you can normally find what you're after quicker than when doing a lot of scrolling down in the app)