For details on Android SDK changes and how old they are; you can refer to:
https://developer.android.com/studio/releases/platforms
If you're not doing anything fancy, testing common gameplay in Android 4.4 is ok.
Once you get to the point on integrating Google APIs or fulfilling
Google Play's quality guidelines, you'd have to move on to new devices (or at least their AVD emulators) to make sure there aren't compatibility issues.
Edit:
Note that having a high target sdk doesn't necessarily mean that your game won't run on 4.4 anymore. That is decided by your min SDK. Android Studio generally handles the backward compatibility for you; In GMS2 we'll only need to make sure that the project's Android extensions won't crash the game in any way.
Technically, the minimum is Android 4.1(API level 16, not listed on
Google site anymore).
The still somewhat used on the market (like 1-2%) is Android 4.4 (KitKat, API level 19).
The safer minimum is Android 5.0 (Lollipop, API Level 21, released Dec 2014).
Ref:
https://support.google.com/googlepl...=637402646062706504-1797308153&rd=1#targetsdk