It feels as if that (vagueness) is exactly the point: they would have to invest time in finding out how the goalposts have moved.

As is, their app got pulled and they said f-it, I don't care enough anymore to invest that time.

Probably target- and minimum- android version requirements in the manifest, which affects available APIs. IIRC they get bumped up again next month.

If something in your life becomes overwhelming, and you are receiving no compensation or recognition for any of your effort .. what, would you say, would be the smart move?

Totally! I had the same experience maintaining an iOS app (eventually abandoned), which is why I was curious and asked the question. Every new version of Xcode produces more warnings, deprecates more APIs, removes previously deprecated ones, and "strongly encourages" changing your code in all sorts of ways. Yuck! I was wondering if the same was true on Android.

Contrast that to Microsoft, who (at least used to) really get backwards compatibility. I bet I could dig out a Win32 program I wrote 20 years ago for NT4 and compile it today, and it would run on Windows 10.

I believe the record in 2015 or so was running Windows 1.01 calc.exe on Windows 10 32bit... Which was first upgraded all the way from Windows 1.01

Uneducated guess: probably the app just work for wide range of devices and versions, like notepad.exe does, and changing ANYTHING is against its value.

And perhaps that kind of app that are well regarded, needs no maintenance, AND not growing X% YoY IAP yadda yadda is unsightly in their eyes, so they remove it.

Google removed it or the developer?