If every renderer rendered every object in the database, the result would be too cluttered to be usable. Renderers only render a selection of features appropriate to their intended use.
The intended use of the default rendering is to aid mappers, and for that purpose it probably concentrates more on things that help the mapper find the correct location well enough to start an editor on them, than on mapping every small object along the way.
I am not a specialist in rendering, but I can imagine that it is impossible to render everything when there are to many different details close-by. However, how is chosen what to render I would not now. https://www.openstreetmap.org/node/4906087863#map=17/47.18133/14.59588 may be too close to the bench, and only the bench is rendered.
https://www.openstreetmap.org/way/494758390 a jersey barrier is probably not rendered when it is a way. When it is a node within a highway it will block routing through that highway I imagine.
Summarizing, not every data that is in the database is rendered.
The map renderer, Mapnik is used on openstreetmap.org, defines what is actually shown.
The developers of the renderer choose what they will visualize when
Not all the latest defined tags are rendered, as no effort will be put in rendering some tags that are hardly used. This is not necessarily the case here.
Actually, I don’t think that it is difficult to render many of these sorts of features. I maintain a general-purpose renderer that shows most of them. Looking at the tags in your list:
amenity=waste_basket
yes
historic=wayside_shrine
yes, as memorial
amenity=bench
backrest=yes
yes (but not explicitly backrest)
barrier=jersey_barrier
yes (now, but didn’t when this post was first written)
emergency=fire_extinguisher
yes (now, but didn’t when this post was first written)
Does yours do a better job of fire stations? Mapnik displays what appears to be a Hamat symbol for highly-flammable substances. I look at Mapnik and I see a flammable liquids store. Or maybe it’s something on fire, like an underground coal-mine fire. Maybe it’s an Olympic Flame. When I see that symbol I don’t think fire station.
I ask because I recently added some here: https://www.openstreetmap.org/#map=19/52.08823/-4.64777. I’m still waiting for the allotment association to get back to me clarifying some plot numbers. Which reminds me, the wiki says to put the plot number in a ref but that doesn’t show even in iD (except in the tag list) so I ended up also putting it in a name and then adding area=yes (without the area=yes it’s treated as a way and the name applied to the border). I expect OSM purists will now yell at me for that. However, the guy who proposed it didn’t use ref when he added his own allotment, so I don’t care.
I’m no artist. I can’t even draw the curtains. But a fire engine (what Merkins would call a ladder truck) seems better suited. Almost iconic, in fact. I doubt I could do a reasonable one in a 16x16 image. I’m not sure that anybody could.
I think you’re overestimating the height of the “barrier to entry” for designing icons on OSM-based maps
Whilst “proper design skills” is something that some people creating OSM maps clearly do have, it’s certainly not something that everyone has (I certainly wouldn’t put myself in that group). There was an alternative to the “standard” map called “Osmarender” that was a running joke for many years - it had many advantages (instant update at a time that the standard map updated once a week, rendering some things that the "standard map still doesn’t), but it wasn’t pretty: http://fakestevec.blogspot.co.uk/2012/03/geosuck-0002.html .
If you’ve got an idea for an icon that might be more representative than an existing one it’s still worth sharing it even if it’s rubbish, because it might give someone with better design skills an idea for how to do it and look nice as well.
I tried to do a fire engine. What I came up with looked like Thomas The Tank Engine. In drag. After he’d been pierced by a carelessly-thrown javelin.
I suspect that if anyone with better design skills had seen the existing fire station icon, that person would already have submitted something that looked far better.
I might have another go at it some day, but I have little hope of producing something that would serve as anything but an example of what not to do.