S&W says NOT.
I don't see that in the Shield 9 manual. However I was reading the manual for my M&P 22 Compact and saw this:
"Press down on the slide stop to release the slide and allow it to carry fully forward on its own under spring tension. Or, pull back on the slide and release it from your grasp. Either action will strip a cartridge from the magazine and seat it in the chamber of the barrel. "
Similar language is in the M&P 380 Shield EZ manual. Non of my M&P pistol manuals specifically state to NOT release the slide by pressing the slide stop.
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