> Unlocking the bootloader is becoming increasingly rare.

This is a bit of "you get what you buy" territory. Google's devices have official bootloader unlock capability and always have, with instructions here: https://source.android.com/setup/build/running - already updated for the Pixel 3a & 3a XL even. You take your risks when you buy something that doesn't advertise that.

> Most of the interesting stuff happens in the backend anyway, so what's the issue.

I think most of the interesting stuff on the device is even in Google Play Services and not in the firebase library. I have no idea why the client SDK can't be open source, but how much does that even matter?

I just bought a new Pixel 3a only to find out that my cell provider locks the bootloader until I've paid for 3-4 months service with the phone, despite previously having an account with them.

This was not mentioned at any time during the purchase and they would not take my phone back.

Unfortunately that's the problem with getting any phone via a cell provider. You are typically beholden to them in some fashion for a few months.

Buying unlocked phones from the start can be an expensive downside, but being free from contracts and carrier bullshit can be a huge upside.

I don't know the ins and outs of this, but wouldn't that still mean that the app would have to rely on google infrastructure to do push notifications? That might be a fix for the licensing problem, but it wouldn't address the problem that that would move virtually all remaining google-free (-ish) messengers into googles direction, with the potential for centrally siphoning of (meta-) data

No, the app can continue to use third party push notifications if the OS allows it to.