What is MoPub used for?

Mopub mediation was deprecated following Applovin's acquisition of Mopub. And all publishers that were previously on Mopub were mostly migrated to Applovin's monetization/mediation platform MAX. Smaato used to support Mopub mediation (until its deprecation) and now supports MAX mediation (for those publishers using MAX). So if you are a publisher, you should be able to tap into Smaato's demand for your monetization needs via MAX (or any mediation platform of your choice).