Sorry again for late reply
PWA is a core base functionality of Discourse, no third party nor plugins.
And as I wrote in my question description, it works on mobile. One can install and use (with sharing from other apps) a Discourse web site as a real app with Chrome for Android. But it doesn't work on Chrome for PC.
But I guess you can not do nothing about that because even meta.discourse.org doesn't install on PC and has one of my errors:
Service worker does not have the 'fetch' handler
I have noticed that Chrome for PC has been much more restrictive on PWA that it was before and any tiny warning prevent the installation process.
More about PWAs => Progressive Web Apps, Desktop Progressive Web Apps or https://pwa.rocks/
All the best