actually no such thing as PWA, there are plenty of web standards but no web standard about PWA
the web standard that governs standalone mode and home screen button and such is a draft (web app manifest)
the website does not need to be progressive, not even offline capable to get standalone and home screen button
the "very dangerouos" service workers can run in any website silently without standalone mode or app buttons or any user interaction (maybe because they are not that creepy after all, an open source javascript that tells browsers how to cache)
I really hate this web app distinction and install language because 1. it is not standard 2. it is not true 3. it is contra-productive
I wrote a piece about this somewhere in a top level comment. Apple breaks web capabilities both in websites and standalone websites (web apps).