There are various classes of upgrades that don't need an outage. For instance a couple lines of css, or a trivial php changes which are more about syntax and fixing log spam. Anything where there is no real impact of old code and new code running side by side on different frontends at the same time. As long as within a single session or series of requests you don't toggle and only ever move forward (eg cache busting url params)
At the moment there is always a glitch during an upgrade when $CFG->upgraderunning is set even when there is no maintenance mode.
We want to get to a point where we have the option to do a full blue / green with two sets of fronts and cut traffic across without any downtime.
Obviously this isn't for all releases, but deciding when to use this is out of scope, this just gives us the ability to run a cli upgrade without the outage, eg
$ php admin/cli/upgrade.php --non-interactive --no-outage
There is a lot of logic scattered around which depends on $CFG->upgraderunning so I don't intend to touch that, I think I'll just make a second $CFG which when set allows requests to be served as normal.