Keeping the finger off the trigger until ready to fire is a comparatively new concept, at least as a safety rule that's taught and enforced by instructors.
Is the 50+ years that I've been interested in firearms "recent" enough?
50 years ago I was helping my father give Hunter Safety classes. It was taught then. I'm pretty confident it was taught before, LONG before, as I said, I think about as long as there have been triggers.
And, there have been people who don't follow that rule JUST AS LONG!
All those old (and even new) movies and TV shows are NOT TEACHING TOOLS, they are entertainment, and as such "dramatic" visual effect often counts for more in the director's eye than ACCURATE safe gun handling.
The old photos are the same, in that regard. They are NOT taken to show safe and proper gun handling, they are either posed photos (where the gun is known to be unloaded and people put fingers on triggers for the photo effect) OR they are candid shots, simply showing how many people DO put their fingers on the trigger, despite what they have been taught.
Why do you think all the military bolt actions (and many semi autos) have two stage triggers??? Simply put, it is because both the militaries and the designers of the time recognized that no matter how much, or how well you teach and train them, soldiers ARE going to put their fingers on their triggers when it "drops in the pot" as well as during less stressful times.
The first stage slack/take up was the way they chose to reduce the risk.
The rule is not new, it has been taught since day one, the only thing "new" is greater emphasis on it, especially with modern firearms.
Are there any stats comparing ADs from (trigger safety only?) compared to shooters with thumb safeties?
None that I know of, however, you can get a general idea from the AD stats with GLocks in their early years, as they were the first common semi WITHOUT a safety in the usual sense. And how the NYC police required GLock to create a HEAVIER trigger pull, after an "excessive" number of ADs with the (then) new pistol.
I see here on the web people often talking about "Cooper's 4 rules", and would like to point out that there are a good dozen or so "rules" for safe gun handling. Taught by the NRA, and (at least they used to be) listed in owner's manuals, etc. Cooper's 4 rules are the "short form", the primary, most important ones when you handle defensive arms. The others should never be ignored, when safety is the primary goal, but they are less important than the primary 4, and may not be practical in a defensive firearm.
One rule is keeping the chamber empty, except when you are ready to shoot. Fine for game hunting (other than dangerous game) but not a good idea for a cop, etc.
The same for opening the action /unloading the gun when crossing a fence or other obstacle. Common sense safety, but not a high priority in defensive gun use, where is where Cooper was focused a lot of the time.
Hardly a fair analogy. More like buying a car that has 4 wheels (no thumb safety) instead of 5 (an extra, superfluous safety) because you don't need 5 to get around.
Sorry, I have to disagree with that analogy. The safety isn't a "superfluous extra" like a spare tire. It is more like the "Park" position of the selector lever of an automatic transmission. It's not "needed" many cars don't have one (manual transmissions, and some early automatics), but it is handy and useful.
The "spare tire" of your handgun is the extra magazine. NOT needed for "getting around" but handy to have, when the one in your gun "goes flat".