The Bell System standard was an extremely high Erlang number, which of course they strove to provide at the lowest cost which meant extremely high utilization rate on hardware which in turn meant extreme uptime (compare to the then contemporary PTT QoS even in major economies like France).

This is also why the software itself was designed with so many internal defenses and what I would consider an “immune system”. I’ve never seen anything like it even on an aircraft control system. That is mentioned in passing in the brief article but is easily missed if you don’t know what it’s referring to.

Most of what is done on the Internet at, say, “layer 5 or above” isn’t at all important so there’s no need for this level of SLA, but the actual backbone carriers do still carry SLAs at around that level. With packet switching it’s easier for them to provide than it was in the days of the 4ESS and 5ESS.