Google rolls out new security features in every version of Android, but some changes only apply for apps that target newer API levels. Thus, Google announced late last year it was cracking down on apps built for older versions of Android. Today is the big day—developers can no longer upload anything to the Play Store without targeting at least API level 26 (Oreo 8.0).
The new policy is fully in effect today, but it's something developers have dealt with for several months. Starting on August 1st, the Play Store stopped accepting new apps targeting pre-Oreo API levels. Today's cutoff adds app updates to the policy. So, even pushing a minor bug fix requires API 26. That means all currently supported apps will have security enhancements like limited access to user accounts. Apps that aren't supported anymore can remain live with their old, insecure APIs.
Going forward, the minimum API level will tick upward each year. So, next year the minimum target will become Android Pie (API level 28). Google also warns that a future version of Android will restrict apps targeting ancient API levels. So, it's smart for developers to bump old apps if they want people to continue using them.