McNick:
Welcome Aboard!
You're going to get letters

....
Just IMHO, the M&P series guns don't really need a thumb safety. Other than the Shield, they're pretty easy to get used to, and if you're into 1911's or other guns that must have a thumb safety, then the one on the M&P won't be particularly strange/different. However, the gun really doesn't need it.
If you bought one so-equipped, and want to remove it, it's very simple, and S&W will give you little plugs to fill the holes in the plastic frame. You probably
cannot add one to a gun you already have. Removing the plugs is the easy part

, but, mostly, it's whether or not the gun's sear block is designed to accept one. If you have a Hilary Lock, it won't....
(The "Hilary Lock" is the key operated "switch" that deactivate the gun. These are necessary in some areas to give the Bad Guy a chance to kill you before you can make the gun ready. OK, one might be useful with kids around for "dead storage", but your "grab it now" gun shouldn't have one. If you managed to buy a Hilary Lock gun, it's easy to remove, too, and there's a plug available

.)
I have a thumb safety on an M&P9C. Probably a bad idea, but I took what I could get at the time. Easy enough to remove - I just have to get the plugs.
The Shield's thumb safety is easy to remove, too. The problem with it is that it's a bear to switch on or off. If you decide to just leave it off, you have to pray that it'll stay that way. I don't think the larger M&P's will stay on or off as reliably if you're depending on it to be in a particular state. (One big rule for defensive guns - the thumb safety, if it exists, MUST work the same way for all of your defensive guns. Up to fire or down to fire, but don't mix 'em.)
Mag safeties are one of those personal things. I prefer to NOT have 'em.... (I have one "serious" gun with one right now, but quit carrying it decades ago.)
Some people like the idea of being able to do a reload while not rendering the gun inoperative. To me, that's one key, and another is just fewer parts to malfunction.
Others like being able to render the gun inoperative by dropping the mag in some kind of fight. I don't think this is particularly wrong, but the fine motor skills necessary to do that while wrestling around may not be there. YMMV....
(It's extremely easy to remove the mag safety from an M&P.)
All of that said, IMHO (channeling Mas Ayoob) it's a bad idea to remove a manufacturer-supplied safety device. Buy the gun set up the way you want it, or
add something if possible.... IMHO, besides some laws in some areas to protect us, "Civilian" use of a defensive firearm probably won't cause any problems if the Judge(s) and Prosecutor(s) aren't seriously anti-freedom. Civil Court is the real hazard, IMHO....
Enjoy the new pistols, and take the time to learn how to use them safely.
Regards,