A while back, Google swapped out the old zoom slider in Maps and replaced it with a plus and minus buttons. The good news is, if you miss that slider, it's easy to bring back.
In Google Maps, just hover over the plus or minus buttons for a couple extra seconds and you'll get the option to show the slider again. Once you do, you setting should stick every time to visit Maps in the future.
Show Zoom Slider in Google Maps [Google Operating System]