It's not an improved "trigger" (assembly). It's a combination of parts & assemblies, some of which are being revised and receiving subtle changes.
You also have to keep in mind that on any given day there may be 15-30+ different people who work customer service phones/desks, and their experience ranges from "very" to "not so much".

Some may have previously worked production, or elsewhere in the company over a 30 year career. Some might be new.
Not everything that's being done in engineering, production or repair might be passed along to the guys & gals working customer service phones.
I've had them end up transferring my calls to engineering or repair so I could get more specific & current info about something (as an armorer).
Other times they've put me on hold and called over to Production to look into something, and have been surprised with the answer they got for me, as it was "new" to them.
Other times I've had to ask the armorer training (academy) or a LE rep to look into something, to try and resolve some conflicting info I'd received.
It's a big company, and not all ongoing changes, revisions, new parts, etc are necessarily being disseminated (at least in a timely manner) to all corners of the company.
The subtle changes in the existing trigger bars for the different models aren't resulting in "new triggers". Not like when the new strikers were the result of significant materials & design changes.
The slide stop assembly change is the neat one, as it increases the tension of the slide stop spring (to help control lever movement, or "flutter", during recoil),
and also helps to give more 'push' to the tail of the trigger bar under the resetting sear nose. I've tried the new slide stop assembly in an earlier M&P, and it does provide just a bit more noticeable 'snap' during trigger/sear reset.
That's nice, I suppose (as some customers have been asking for it). However, I don't shoot to "trigger reset", myself, but to trigger
recovery, and I'm not one of those owners/shooters who want to duplicate the connector slap of another make of plastic pistol in everything else I own, shoot & use.
I haven't exactly been trying to keep up with all the subtle changes and nuances of the ongoing improvements to the M&P line. Not unless it involves the
need to start replacing parts in working guns, or having to be able to distinguish between earlier & newer parts & assemblies when servicing guns (like the change in the sear plungers & springs, and the sear blocks in which they fit).
I was told last year that S&W's engineers were busy experimenting with different off-the-shelf M&P components to try and address some customer feedback issues, and maybe making some minor changes to existing parts as an improvement in design might be determined. (The MA complaint sear block, with it's heavier plunger & spring, became a "standard" assembly in this way, BTW.)
Although we were told in my last class that the PC sear would be part of the subtle improvement in the newer guns, but NOT with a lightening of the trigger pull weight, the PC sears are still not available as spare parts. Getting one as an armorer is still a bit of a difficult trick, as they're carefully monitored inside the company (I don't have any, but I haven't really tried, either

).
I don't have a list of the interchangeable parts & assemblies for the Shield versus the rest of the M&P pistol line yet. I'll wait until they have approval for their Shield armorer class material. (And until I have the time to take one apart, and not just spend an afternoon running one through drills on the range.)
In the meantime, I've found the newer M&P's to offer pretty decent "triggers". Some more than others (but there's been an allowable +/- 2lbs in the triggers of the stock guns since their introduction). The Shield 9 I used had a pretty decent trigger, but so did the last couple of M&P 9's I tried (compact & VTAC).