No such thing, but some are better than others at the job they are designed for.
The light travel sling bag has been on the radar for awhile. I know as well as anyone who shleps gear all day that even a few kg on the shoulder seems to double in weight over a day. Heat or cold force tough decisions, as do places where going unnoticed and out of the way is important.
Shoulder bags have their place, but travel is not one of them.
The TT Turnstyle 10L (chosen as the best fit for me out of a 5L and 20L) in Charcoal fits almost all of my needs perfectly.
The main compartment holds bigger M43 gear (EM1 with 12-40 or 12-100), a second body with/without mounted lens and a third lens option. All are in the “ready to go “ configuration, so not reversed hoods or caps needed.
Overall the bag feels better finished than many others I have tried and has the feel of long-term sturdiness that is reassuring. It is also nicely classy in a low profile, modern street kit way.
Not only is this bag ideal for a small kit over long distances, but it manages to become effectively invisible when not in use.