Not necessarily that it's not done any more, but more that the reason has been long obscured.

So the U.S. Army's regulations forbid carrying of umbrellas in uniform, leading to no shortage of bellyaching from new guys caught out in the cold, driving rain in uniform. You know why we have this stupid rule? As Jonathan House explains, it's because von Moltke the Elder was so damn good at his job. After German success in the Wars of Unification, everyone wanted to emulate the Prussians, and no Prussian officer would be caught dead carrying an umbrella. What are you, middle-class? Officers carry swords.

I'm struggling to find it, but the first essay I wrote as an undergrad was titled "Touching the Penis in Islam". It turns out that it's a rather complex question because certain forms of touching straddle the line between whether they would require performing the major ablution (ghusl) or only the minor ablution (wudu) prior to prayer, whether the touching is incidental or carnal, and whether in each respective instance the specified ablution is merely "recommended" as opposed to being obligatory or unnecessary.

Slightly annoyed that I can't find the actual essay with the relevant citations but you can see an example of this kind of thing in this religious Q&A document: https://islamqa.info/en/82759

For context the "Imam Ahmed" in the first opinion is Ahmed ibn Hanbal. The second opinion, as I understand it, is Hanafi.

In a completely different form of etiquette, I was really amused to learn that in traditional "hall" settings it's considered perfectly acceptable, if you're in the middle of a long row with your back to the wall, to simply walk over the table in some settings. Oriel College, Oxford is an example where I understand this has long been the case. One of those instances where you might expect high society to go all in for ultra-correct table manners when in reality they're just as likely to start drunkenly walking over tables.

In 10th century central Europe (among the Franks, for the most part, although you find it elsewhere too), there was a punishment called harmscar. This was usually applied to noblemen for breaches of agreement with their superiors, and took the form of having to carry a dog or a saddle for a few kilometres. This seems like such an incongruous punishment - and was applied for crimes that we'd call treason - that it must run into local concepts of how things were properly done.

So if you're a 10th century noble in these cultures, it is important never to be seen carrying a dog or a saddle.

For etiquette there should be at least one Louis XIV French example.

In 1715 a servant decided he knew enough manners to pass off as nobility, called himself the Marquis de Ruffec and started on a journey through the south. He'd done quite well, dined in a lot of the great houses, until the Commandant of Bayonne noticed he was using a fork to help himself to the olives. This clearly marked him as an imposter, and he was arrested after dinner.

Most people nowadays don't have any exposure to snuff. When I say snuff I don't mean what you're thinking. Nowadays it's common to refer to a variety of tobacco products that are put in the mouth and ingested through the gums as "snuff" (dip, snus, etc). But these are not really snuff. Snuff is a tobacco product that you inhale. Popular in the 18th century, and most known for the decorative snuff-boxes, it's now a very unusual way of consuming tobacco.

One of the last bastions of snuff was in Jewish communities. Religious Jewish smokers have a bit of a problem. On the Sabbath, it is forbidden to light things on fire (along with a bunch of other things), which would prohibit lighting up cigarettes, cigars, or pipes. While there are some clever workarounds (it was once common in the Middle East to use a hukka/nargilah on Friday afternoon, fill the inside with smoke, and inhale the smoke on Saturday, but that was controversial), smoking is pretty much out. Dip was not around, and spitting in synagogue would be disrespectful. So what is a person to do?

Snuff is the answer. It is easy to carry in your pocket, easy to have a bit before or after services, and doesn't break any laws of the Sabbath. Until the last few decades snuff was quite common among Jews in synagogues, and even now many congregations still have people who use it. Particularly in more traditional communities, where there are more smokers, especially in Israel, where smoking is more common in all population groups than in the US.

But what's the etiquette? The etiquette is that you must offer other people snuff if you're using it. It makes sense--someone could be suffering from nicotine withdrawal. And people who don't need it are unlikely to mooch. There are stories which, as a plot point, have someone be (usually wrongfully) given the huge middle finger of not being offered snuff after services.

Since I just got done talking about Courtly Love in medieval England, here's a fun fact:

It was considered polite for troubadours never to mention the husband or children of a married lady if she were the subject of a courtly love affair.

Water bowls on the table in front of British royalty--actually, so far as I know, this is still "not done", but most people would not realize it's a Jacobite relic. Jacobite supporters and "crypto-Jacobites" would raise their glasses over the water bowl when toasting the King, to change the toast to one for the King "over the water"; i.e., James Edward Stuart.