I could be wrong, but I think Google had made the announcement that NPAPI plugins would no longer be supported sometime last year. They have been phasing these plugins out ever since.
Windows users: there are plugins for Chrome that allow Chrome to use one's native IE rendering engine (and therefore Active X plugins) inside of Chrome. If you search the store for IE Tab, you'll find multiple plugins (presumably with only minor differences), which should allow you to start using Siverlight, unity, etc. in no time. Here's one you can try.
As for the claim that "[PPAPI] is also how the Adobe Flash plugin is implemented in modern browsers.", that is obvously not true. As pointed out, neither Firefox nor Internet Explorer have ever supported PPAPI. So really it's just Chrome/Chromium and derivatives that are using PPAPI, not all modern browsers. Adobe is still maintaining the Active X and NPAPI versions of flash.