It’s is very uncommon nowadays- and there are reasons. One of the biggest reasons would be safety. Most modern cars have airbags mounted in the seats and the doors. Placing an armrest between the door panel and the occupant not only adds a rigid solid object for the human body to collide with in a crash- but also would prevent the door mounted airbags from doing their job since the air bag wouldn’t be cushioning you- it would just hit the armrest.
If you look around modern cars you’ll notice some things on most of their interiors.
>Belt lines (where the bottom of the side windows start) tend to be much higher- even y’all folks will find it uncomfortable to try and rest an arm on the window sill in most newer cars. Metal is generally better crash protection than glass- and with more “cross over” or “SUV” and other talk vehicles being common- cars need protection higher up so in a collision these giants don’t just take the roof off and crush everyone inside by missing the crash structures in the body.
>Door panels and roof pillars are thicker. Extra crash bars and air bags mostly.
>most newer cars have DEEP dashboard to front windshield distance and usually a steeply rakes front windshield. Not only is this theoretically aerodynamic- but it helps keep your head and body from slamming the windshield in an impact, and lessens the odds that any objects penetrating the windscreen will contact occupants.
>hood and fender lines are higher. (This is technically exterior- but can often be seen from the interior.) as mentioned before this gives more metal and protection against vehicles that are higher off the ground. It also is in part for pedestrian crash safety to accommodate crumple zones for pedestrians.
>it’s rare to find anything sharp, and most things are plastic. Look around. You likely won’t see “knobs” on the tops of doors. Protruding toggle switches are gone, or have a rounded “cage” around them (like Mini Cooper center switches). Turn signal and wiper stalks tend to be short, thicker, rounded, and stubby. They’re usually at least partly “shrouded” by the steering wheel or trim. Control switches and buttons like door locks and windows or mirrors tend to be flat or near flat and relatively smooth. Door handles are usually rounded and recessed. Things like “coat hooks” retract into the body, sharp lines aren’t generally seen and dashes tend to be rounded or sloped.
While there are stylistic elements involved- mainly this is for occupant safety. Observation of crash injuries and fatalities on older cars showed that people often suffered injuries ranging from mike to severe such as bruises, punctures, lacerations, etc. not from another car entering the passenger compartment- but simply from their bodies contacting sharp corners, metal, and protruding objects in the cabin. To increase crash safety manufacturers have largely moved to designing interiors to minimize any possible contact in a crash, and to minimize any injuries that might result if one did contact a part of the interior.
>Belt lines (where the bottom of the side windows start) tend to be much higher- even y’all folks will find it uncomfortable to try and rest an arm on the window sill in most newer cars. Metal is generally better crash protection than glass- and with more “cross over” or “SUV” and other talk vehicles being common- cars need protection higher up so in a collision these giants don’t just take the roof off and crush everyone inside by missing the crash structures in the body.
>most newer cars have DEEP dashboard to front windshield distance and usually a steeply rakes front windshield. Not only is this theoretically aerodynamic- but it helps keep your head and body from slamming the windshield in an impact, and lessens the odds that any objects penetrating the windscreen will contact occupants.