Basically, what the title says. Do you use any app, that is proprietary, but either has no OSS alternatives or they’re all not good enough? If there is an alternative, what keeps you from switching?

  • Official Signal app. It asks Google to check Signal servers for notifications and to send them to you if there are any.

    I don’t think that’s how push notifications work. It would not be instant, and very frequent polls are very resource intensive, even more if you had to do it for the servers of a million different apps.
    UnifiedPush has a diagram here, and the popular proprietary push providers should work the same way too.