Besides what MattJ100 wrote above, by itself this is also not true. You can tell Android to not optimize Conversations and allow it to run in the background. It will work just fine with a (usually idle) TCP connection. But for the typical user, having to instruct them to do this is cumbersome, and integrating with Firebase is easier. It does come with its own concerns w.r.t. privacy, both for the payload as well as metadata exchanged.
Hence the suggestion for PagerDuty. It handles all this, because responders set their notification methods (phone, SMS, e-mail, and app) in their profiles, so that when in trouble nobody has to ask those questions and just add a person as a responder to the incident.
As far as I know, Facebook Messenger's XMPP support was only exposed to clients (c2s), not to other XMPP servers (s2s). Microsoft's Lync (now known as Skype for Business) supported XMPP federation until 2019. WhatsApp's client protocol was originally based on XMPP. I'm reasonably sure that both AIM and Yahoo had XMPP server-to-server support ready around 2008, but can offer no proof.
Yup, and the old bridge will keep working locally, still has an open API, and there was an upgrade program when v2 was introduced, with a 33% discount. Four or so years ago.
The original article (https://www.washingtonpost.com/nation/2019/07/26/it-snuck-up...) includes links to some interesting background information, including the event leveling a huge forest area. And it correctly uses square km/miles instead of "square acres".
I hope they pin on the key, not the certificate. For a mobile app I worked on, I had it pin the public key on the leaf certificate and indeed it would fail to connect in this scenario.
For mobile apps, though, you can bootstrap HPKP with a key built into the app. I worked on an app doing this, and it would certainly fail to connect in this scenario.
For U2F there's nothing to be in sync: each key is added individually, and you don't have to add all of them at once. I.e. if you register the key on your keychain at work, you could later add the backup key in your home vault.
For storing TOTP keys on your YubiKeys, those must be the same, so you probably have to add them at the same time, or take a picture of the QR-code before you complete the registration.