1. The log-replication method of upgrading can be performed using the built-in logical replication facilities as well as using external logical replication systems such as pglogical, Slony, Londiste, and Bucardo. Most of which have existed essentially forever.

2. Failovers of any database are not instant, but they are indeed quick! So let’s not claim that you can do an upgrade with zero downtime.

3. In-place upgrades are extremely fast and you can test the speed using a physical replica before hand, usually it’s a couple of seconds though the docs say minutes.

4. MySQLs major version being in the minor position is exactly the kind of “you should be sure you know what you’re doing but we won’t make it obvious” territory that I really despise.