It still happens to an extent. Within your average western infantry platoon the platoon leader has had a mantra of lead from the front rammed through his head since the day they entered officer training.

However, the fact that as they progress through the ranks they will find themselves in progressively less dangerous situations is not a change in battle morality or something like that it merely reflects the current reality of warfare brought about by, largely, technology.

I would argue that it ended with the Napoleonic Wars, mainly because of the size and danger that was associated with command from the front or at latest the American Civil War (see Stonewall Jackson).

The problem with leading from the front is that good commanders would often die. During the Napoleonic Wars, the French had problems keeping Division level generals just from the fact that they would lead from the front and die from the front. At this time, bravery was as valued as tactical skill, but bravery was easier to show than tactical skill, so a brave soldier could find himself going up the ranks.

However, even the highest ranking commanders would lead from the front. Michel Ney is known for being the last Frenchman in Russia, Charles Oudinot was happy to show off wounds he got from leading during the Napoleonic Wars well into his old age, Lannes once charged at a fort in order to make his grenadiers attack. Generals would lead from the front but the wars changed and became deadly.

There are different types of generals. There were those that rode in to battle with their troops and with every swing of his sword, morale would intensify. But there were also those that sat behind the lines, those who overlooked the entire battlefield and try to find the best way to destroy the enemy

In Europe the last leaders that I'm aware of who personally led their armies from the front fought in the Great Northern War, those being Peter I of Russia, Augustus II "The Strong" of Poland/Saxony and Charles XII of Sweden. Of those, Charles was the most aggressive and would actually lead cavalry charges, and was later killed in battle during a siege (possibly by 'friendly' fire, although that's another topic.)

Charles was certainly an inspiration to his men, until he got shot in the leg and almost died. That put a bit of a damper on Swedish morale and probably cost him the campaign when the Russians destroyed his army at Poltava.

Elsewhere, Ethiopian kings were leading cavalry charges into the 19th century. Yohannes IV was shot in a battle with Sudan around 1889, Tekle Giyorgis II before him was wounded and then captured by a rival for the throne while leading a charge in 1871.

The practice died out with the development of ways to kill people accurately from far away. Muskets got more reliable and accurate through the 17th century and with the widespread adoption of linear tactics and bayonets by the end of that century, putting your leader in the front line became a good way to head home without a king.

Monarchs in Europe would command battles through the early 19th century, though. Frederick the Great and Napoleon both were present at a great many battles, although they didn't personally take part in the killing.

The other major factor that hasn't been mentioned yet is communications. To command effectively, any general needs to be able to get information quickly - to see a picture of the battlefield - and issue commands quickly. Until the advent of communications such as telegraph, telephone and radio during the 19th century, the best way to get an overview of the battlefield was to find a high vantage point, and then send orders to regiments by horse riders. Once the communications systems mentioned developed, generals moved further behind the lines.

Generals in World War I are often criticised for sitting in their safe HQs miles behind the lines while the soldiers endured the trenches, but in reality this was the best place for them - certainly during an actual battle. They were at the heart of their information network, able to receive and collate information, form a coherent picture on the battlefield and issue orders accordingly. They couldn't do this so easily from a shell crater in no-man's-land.

The problem that occurred during the First World War was that the moment the soldiers left their trenches they were out of communication until they could lay some telephone lines across no-man's-land. The advent of radio communications during World War 2 allowed commands to be given throughout an engagement, which enabled more junior commanders to follow the example of the generals and drop back for a better overview while remaining in touch with their men.