Sounds similar to Active Fuel Management - or something like that, came with the 5.3L GM V8 beginning maybe 15 or so years ago. The engine would run on less than 8 cylinders under light load conditions. Many threads in forums and elsewhere how to deactivate it. Caused more trouble than it was worth. My 2002 Silverado fortunately is too old for it.
With the Subaru system - is there just one starter? I've heard of some engines with stop/start that have a secondary one
specifically for the feature. If there is just one, I'd think it would wear out prematurely.
....yup, GM uses AFM and Dodge calls theirs MDS (Multi Displacement System). Dodge has been using MDS on some their 5.7 Hemis for the last 15 years or so. In fact, I have the system on my 2010 Challenger R/T.
This is one of those fuel saving features that really doesn't bother me in my particular application. To be honest, it's really not noticeable at all except for a very faint change in tone of the exhaust while cruising. However, this change in tone can become a bad droning noise with the wrong aftermarket exhaust setup which is why those with the Merc Nag1 autos (like me) have to choose wisely or use one of several methods to disable the MDS system like putting the tranny into manual mode. Corsa and Borla make systems that work well with my car that create little or no drone while MDS is active.
Honestly, the MDS system only really makes a difference while on longer trips on the highway, etc. .....but even then (IIRC), it's only a 1 MPG or so gain. Don't get me wrong though, it's a welcome savings on long trips to Vegas, etc. especially since it's really imperceivable (can't feel it and can barely hear it if at all). Best of all responsiveness is seamless and instant when all 8 cylinders are called into play. IIRC, my last trip to Sin City I got 25+ MPG without even trying. ....traveling 80+ most of the way.
Subaru's Auto S/S system is a whole different story IMO / IME. In our scenario in my wife's 2020 Forester, it only comes on maybe 50-20% of the time while at a light and when it does, it only serves as a reminder that I once again forgot to shut it off before I left the house. The car shudders, the a/c fluctuates and I just find it annoying. .....especially for the marginal saving in fuel cost to us.
Either way, I'd say a Forester isn't the kind of car I would even consider if I placed such a high priority on fuel economy, pro-regulation and/or doing all I can for the environment. There are vehicles on the market that are far more responsible than Subaru in that regard. Personally, I'm not the kind of person to give a second thought to keeping the engine running while waiting for my wife in the store, etc. on a hot day. .....or starting it remotely to let it cool off before we get in.
My wife and I chose the Forester for her in '08 and again in '20 for its utility, fun (e.g. off-road capabilities, etc.) and other features. The modestly better fuel economy we have been experiencing in her '20 vs. her '08 is nice though. as I'm obviously not trying to go out of my way to spend money on fuel just for the sake of it and since this will be her commuter, the difference (while not huge) will still be enough to notice.
I hadn't heard anything about a secondary starter. AFAIK, it's just your standard setup in that regard. As for wear and tear, the engine, starter, etc. goes through far less stress simply idling than it does constantly restarting. ......but whether that will translate into starters failing more often, etc., who knows.