As an engineer, I get pissed whenever I see 100% uptime, or eleven-nines, nine-nines, or other impossible targets. Like, how am I supposed to design a system with numbers like that?
100% uptime doesn't necessarily mean nothing failed, it means the failure detection and mitigation worked within the allowed windows. In a typical internet environment, that means allowing connections to die when the server they're connected to dies. It's would be possible to handoff tcp connections, but nobody does it.
If you want to get close to those numbers, you need to have a real reason, and then you need to make sure you have a good plan for everything that can go wrong. Power, routers, fiber, load balancers, switches, hosts, etc. And then do your best not to push bad software / bad configuration.
Bare metal on quality hardware with redundant networking goes a long way towards reliability, once the kinks are worked out.
Nothing wrong with just using SLOs, but if you are a technical lead or senior engineer, you should have the big picture.
Or - if you prefer - what is the "reasonable" percentage of issues - timewise - for an internet service?
Agreements to uphold past performance are much better.