Our product is a XMPP client, which can't work if it is not actively running in the background, so we were working with making apps run in background on Android for over 10 years, since version 2.0.
The trend is clear: with every update, we get more restrictions. Not only bg services are killed off from memory by aggressive memory managers of crappy phone vendors, Google themselves push developers to rely on proprietary FCM services. The writing is on the wall, and one day background services in Android will simply cease to work, all in the name of better battery life and customer care. So this OpenPush service will be short lived, it'll likely be disallowed to work in this form in Android 12 or 13.
The only proper long-term solution here is an anti-monopoly legislation, that would make Google allow Android users to subscribe to a third-party push notification service. Such service is, actually, a relatively simple: current FCM is just a modified XMPP connection that a phone keeps, and it will be easy to replicate and deploy.
Apple is guilty of this, too. A proper anti-monopoly legislations should target them as well, and not only make them allow third-party app stores, but third party push notifications as well.
This control of push notification services and restriction creep that occurs on both platforms makes Google's and Apple's grip on their respective OSs much tighter than Microsoft's grip on Windows ever was. This tyranny must be stopped, and I don't believe it can be done without legal action, by technical means only.