That’s a difficult question to pin down since it varies among different cultures and civilizations.

In the context of the ancient world, kings fighting on the front lines was extremely common during the Hellenistic period. Alexander the Great was perhaps the most famous (or infamous), and the same can be said for the Successor Kingdoms of the Antigonids, Seleucids and Ptolemies. Macedonian Kingship was made or broken on the battlefield, leading a decisive cavalry charge (usually), and a disproportionally high number of Hellenistic kings died in battle. Contrast this with contemporary and later Rome, as the Emperors and generals/subcommanders rarely are described as being in the thick of the fight. A few examples are here and there, such as Julius Caesar seizing a shield to rally his men, or the Emperor Julian against the Sassanid Persians (and died due to a spear throw), but by and large this wasn’t the majority of time.

We have instances of kings fighting on the frontline up to and including the Great War (eg Albert I of Belgium). The problem with kings fighting on the front was that it tends to create major issues when a king killed on the field of battle, often leading to a rout of that army in a panic, but more importantly it can plunge the realm into a succession crisis depending on how far away the king is from home and whether or not they’ve already planned ahead with an heir. It’s frankly amazing that someone like Alexander the Great didn’t die sooner due to how many times he threw himself in the thick of battle. It was still a problem because having a king on the front could be a boost to the morale of his soldiers (eg the famous anecdotes of Alexander, Pyrrhus and Caesar’s participation bolstering their troops resolve), but it also was important in validating your right to rule, as victory either literally demonstrated your skill and bravery or it proved you were under favor of God/the gods. Workarounds could be made, such as having subcommanders or generals fight in your name, but what could happen is the successful general uses his military victories and popularity to launch a civil war, as would happen so many times in the Roman Empire.

TL;DR It fell in and out of favor depending on the culture, and really only stopped after WW1 when many of the monarchies had lost much of their power or were abolished outright.