If you add a second page of apps, go into jiggle mode, tap the page indicator dots at the bottom to enter the overview mode, then uncheck a Home Screen. It will still exist, but it will be hidden.
Okay I’m really glad I asked because I did not know about that feature at all, and it does the job even if in an awkward way (adding a second PWA creates a third page when the second one is hidden). The PWA I was using to test iOS/PWA interactions now shows up in the App Library and Spotlight among the app results.
It’s not great, but as compromises go this one is basically reasonable, though Apple should still just let me dump them straight to the App Library. Thanks!
My understanding is that being able to hide Home pages predates Focus modes as it was the poor (and hard to discover) solution to what the Apps Library became (for apps that you needed installed but didn't care to see all the time).
But yes, Focus Modes definitely elevates it to another level when you've got Mode-specific pages.