In civil airplanes, they’re not normally installed other than for Mx or an emergency, and are kept onboard. Still, they need to be ensured on preflight that they are, in fact, not installed. I’ve never seen them installed on the 737 I fly, but I‘d definitely notice them if they were, as I specifically look at those pinholes on the struts during the walkaround, both since it’s a preflight item, as well as its one of the few things near eye level that can actually be checked externally.
The showing the pins thing is more for mil, where an aircraft has gear pins, as well as pins for other things like racks, etc. And also for pins that are stored externally on the aircraft such as in a stowage box in the gear well. Again, not common for civil ops, nor necessarily needed. My only point is that if pins do happen to have been installed, that they should be noticed on a normal preflight, especially with streamers attached to them (and assuming that someone didn’t slip them into the gear after the flight crew already preflighted, for some reason). I’m not advocating that they must be installed for normal ops.