Once you get to 99.9% uptime you’re still less than 1 hour per month. I used to run apps for companies at work and it was standard operating procedure to have two app servers + load balancer for upgrades.

Drain a server, upgrade it, boot it, check that it works, let it accept traffic. Repeat once more. Done.

It may be true that most businesses don’t need the uptime, but I’d say it’s usually easier to give them high availability rather than field complaints when some random user tries to look something up during a maintenance window.

But where does the data sit? Would you be using some fancy multi-master synchronous replication for that, or is there a database server behind it?

A pair of database servers (leader and hot spare) behind it, although some data was in-app and shared between the two app instances (or more; two was the minimum).

I don’t think of this as particularly fancy or expensive, considering how much you have to pay for engineers and technicians. It was critical enough for >3 nines, and that means having a technician on-call.

I saw this setup at two different companies I worked at, even though the tech stacks were completely different. Whenever I price out setups for similar requirements in cloud I almost always end up with basically the same setup, just on cloud VMs instead of dedicated servers.