Holding your arms up all day isn't ergonomic.
For an idea, look to pianists, professional transcriptionists, or 1960s secretaries. These trained professionals have no problem using their hands in front of their bodies for hours every day, with no palmrests in sight.
You don’t
need custom designed furniture. You just need to sit up straight, bring the keyboard close in to your body, adjust the keyboard tilt until it aligns with your forearms, and let your upper arms stay loose at your sides. The key is that your shoulders should be relaxed: the same position they would be if you were just standing somewhere.
Note that we’re just talking about posture while actively typing. If you’re just sitting in front of your computer reading a blog post on the screen, by all means, set your arms and hands down somewhere.
It would be closer to ideal to have a taller chair and/or lower desk than most people currently have, with the elbows at a >90° angle, but even with a low chair and high desk, by tilting the keyboard up at the far side, it’s possible to type just fine. The elbows will have more bend in them, is all.
If we really wanted to get closer to ideal, everyone would have a split and substantially tented keyboard, with some space between the halves. Again though, while that would be better, it’s entirely possible to type on a standard one-piece keyboard, just with a bit more wrist pronation.
Anyway, if you find that armrests, palmrests, etc. etc. help you, don’t let me stop you from buying and using them. Do whatever you like.
It's kind of a wild supposition to say because one wants arm support then they have screwed up posture.
The vast majority of westerners, I’d say at least 90%, have screwed up posture, from a lifetime of sitting on poorly designed furniture all day long. This includes me a good proportion of the time. It’s possible to sit better on standard simple furniture, but it takes a lot of practice, and it’s pretty easy to fall back to slouching. Better furniture helps a lot.