Likely constant notifications such as 'to use [app name] you need to update/enable Google Play Services'
You can turn all those notifications off. Many apps, too many in fact, tend to depend on Play Services for things like in-app purchases, even if you never use them. As such, it makes them error out and put a notification up, even though they can technically work fine without, so long as you are ok with not getting the premium content and just use the free option. Some however, like games, tend to put a full-on pop-up demanding you enable Play Services while the game is still running behind the alert, and close out if you say no. sometimes turning off the screen overlay permission stops those, often just tapping anywhere around the alert dismisses it, and sometimes such a game will just outright not run at all, like Pokemon Go.
Back in the Android 2.3 days this was never a problem. Other than UI design preferences, I use old apps apks to replace any 'modern' app with an Android 2.3 counterpart, and never deal with Play Services at all. AOSP Music, Angry Birds 1.6.2, ES File Explorer 3.x or below, Gallery GB, and Dolphin Browser Mini from 2009 don't do any of that. the awesome thing is that on modern devices, that 2009 browser can load modern sites since it seems to be using the webview engine that Chrome does, so it future-proofed it.