André Labonté, Project Manager on the Material Design team, directly answers this question:

“Stackable widgets are possible on some Android manufacturer’s devices and this is something we are looking into for AOSP. We getting this request a lot. So stay tuned. We aren’t committed to this yet, but keep asking and we’ll work on it.

OnePlus and Samsung are the Android manufacturer’s that André was mainly referring to, but there are many more across the eastern hemisphere. Having it built directly into AOSP would not only make it available for Google Pixel, but it would free up Android developers from these manufacturers to work on other things. The pipeline seems to be that when things that were once only available on al specific manufacturer devices become available across all of Android, those same manufacturers find something else to be a catalyst differentiator among devices. It’s a health progression.

I’ve been using stackable widgets on my Pixel through the Smart Launcher and it’s been pretty great. I can see the appeal, especially since the general philosophy over widgets is to be an app instance that does one thing really well instead of something that is a full window into the app. Though iOS birthed this new resurgence for widget design, Android continues to provide better functionality and practically on having widgets on the home screen. Soon widgets will be on the lockscreen on mobile.