It's a commonly-held notion in the shooting community that various mechanical safety doodads and gizmos are recent additions to the American firearms scene, driven by anti-gun legislation and an industry fear of lawsuits. However a quick study of the past will show that it just ain't so.
As a matter of fact, even in the late 19th Century, safety was a big advertising point for firearms in a rapidly-urbanizing America: Both Iver Johnson and Smith & Wesson touted the safety of their small revolvers in advertising, and by the early 1900s, Iver Johnson was using "Hammer the Hammer" as an ad slogan.
When automatic pistols debuted on the commercial scene in the early 2oth Century, they were quite a novelty. The early full-size Colt holster pistols had a rudimentary safety in the form of a pivoting rear sight, but this was soon dropped and the pistols were without any safety at all other than the exposed hammer. Less expensive pocket pistols were another matter, with both of Colt's small pocket auto designs from John Browning featuring a thumb safety and a grip safety from the start.
Savage's Model 1907 .32 had a positive manual thumb safety as well as a mechanical loaded chamber indicator, which consisted of a pivoting tab that raised up to indicate a cartridge up the pipe. When Harrington & Richardson entered the pocket self-loader game in 1914 with its modified Webley design, the pistol sported not only a mechanical loaded chamber indicator, a grip safety, and a manual thumb safety, but also an automatic mechanical safety that prevented the firearm from discharging when the magazine was removed. Colt engineer George Tansley immediately came up with a magazine disconnect that was fitted to the company's Vest Pocket models just two years later.
The fad for the more Rube Goldbergian devices was a fairly brief one, however. Savage disposed of the mechanical loaded chamber indicator, and only the first series of H&R autos have the magazine disconnect. What caused the popularity in the first place?
Lacking a time machine and without reading any periodicals of the era (although the topic has intrigued me enough to want to dig further), I'm going to hazard a guess: In the early 1900s, self-loading pistols were a novelty; even people who had extensive experience with handguns had had all of that experience with revolvers. Compared to a revolver, the manner of clearing and safing an autoloader is not an intuitive process. Probably the single most common cause of negligent discharges among novice self-loader users is dropping the magazine after clearing the chamber, rather than before. They've seen the round fly from the chamber, and therefore the gun must be "safe", right? And in 1900, almost everybody was a novice self-loader user.
The solution, of course, is training and experience and not more complicated fiddly little parts on a gun, and for the most part magazine safeties went away. They remained popular in one segment of the autoloader world, however: Every day, police departments and military organizations around the world hand out guns to countless people, many with only the most rudimentary of handgun training. And at the end of their shift, these same people are expected to come back in and safely turn in an unloaded weapon without shooting themselves, their armorer, or their fellow gendarmes or gefreiters. In this setting, magazine safeties retain their popularity with many issuing agencies and armies, since sending all their personnel to Gunsite would be prohibitively expensive and time-consuming.
Of course the belief that mechanical gizmos can substitute for safe handling has penetrated various legislatures and courtrooms, and more and more guns are fitted with these Rube Goldbergian contrivances in an attempt to remain salable in as many jurisdictions as possible. We can only hope for a brighter tomorrow, when we look back on this era of mandating hardware solutions to software problems and laugh.