welcome to TFL
I am sure there is a simple answer as to why no SMG's have this.
The simple answer is that it serves no purpose.
Although mechanically very simple, open bolt SMGs are widely misunderstood by people who have not been trained on them.
I have personal experience with the US Thompson, and M3/M3A1 "greasegun", the British Sten and the German MP38/MP40, (which people incorrectly call a "Schmeisser") as well as several post WWII designs including the Mac 10 and the Uzi.
When you say "front notch" I am assuming you are referring to a front version of the inverted "L" shaped notch in the receiver tube that the Sten and the MP40 have at the rear?
Such a notch would allow the bolt to be locked in its forward position, but would not make the gun significantly safer in use though it would technically increase the drop safety of the gun very slightly, it also introduce a slight increase in cost and complexity of manufacture, bue more importantly, could be a complicating factor getting the gun into action in combat.
As mentioned, "drop safety" was not a high priority of gun design during those years, and the firing pin blocks used in handguns these days simply cannot be applied to the open bolt fixed firing pin SMGs.
The only time these guns have a round in the chamber is when it is being fired. Locking the bolt forward on an empty chamber doesn't do much to increase the safety, except in one very, VERY rare situation.
And that situation is, loaded magazine in the gun, bolt forward (chamber empty -the only way this can be done) IF the gun is dropped (or shaken violently) in just such a manner that the bolt comes back enough to pick up a round from the magazine, BUT NOT ENOUGH to engage the sear and hold the bolt back (a very small distance further) then the gun will fire when the bolt goes back forward. This is a very, very small possibility, and while possible, it was generally not considered a significant risk by the people designing the SMG, and most designs don't have anything that specifically prevents that very rare occurrence.
There is one I know of, that does, and that is the US M3/M3A1. And the Greasegun doesn't use a notch in the receiver tube or bolt handle, at all.
Generally speaking, open bolt SMGs are carried "on safe" with the bolt locked back. There are two common ways the bolt is locked back "on safe". One is physically locking the bolt from moving usually done by "locking" the bolt handle into a notch where it cannot move until the shooter moves the handle out of the notch.
The other way is to use the sear of the gun, which holds the bolt back until the trigger is pulled. These guns use a safety (switch or grip safety or both) which prevents the trigger from being pulled.
The Tommygun, the Uzi and the Mac 10 guns use that kind of system. The STEN, and the MP40 physically lock the bolt handle in a notch so pulling the trigger does nothing. The Greaseguns also physically lock the bolt from moving, but use a rather ingenious method (since there is no bolt handle) locking the bolt from moving using a tab on the ejection port cover. And, the designers were smart enough to add a second hole in the bolt so that, with the cover closed, the bolt is locked in either the forward or rearward position.
Additionally, the idea of an SMG going off when dropped and "running away" firing until it runs out of ammo is pretty much movie BS, UNLESS the gun is broken (or specially rigged to do that).
IF the gun is not broken, full auto guns will only run full auto when the trigger is pulled and HELD TO THE REAR. If the trigger is not being held to the rear, the gun and something jars it enough to let the bolt go forward, it will fire ONE round, and then the bolt will be caught and held to the rear, by the gun's sear system, unless the gun is broken and that system is not functioning.
Open bolt guns work almost the exact opposite of regular guns, and require both training and due diligence on the part of the user to work safely. I have personal knowledge of one accidental discharges (actually a negligent discharge) due to a new user not being familiar enough with an Uzi.
This happened at a US nuclear facitity!! The Tactical Response Team portion of the guard force, which was equipped with M16s issued some Uzi's to a portion of the force, (shorter and handier inside buildings) and while they were trained, ONE guy wasn't trained ENOUGH!
According to the wording of the accident report he was "giving an informal class" on the gun (aka "showing it off") in the facility lunchroom
eek
, at the end of which, he "returned the gun to service" by doing exactly what he did with his M16, bolt back, inserting a loaded magazine, and releasing the bolt to go forward. At which point, naturally the Uzi fired!
ONE round, into the lunchroom ceiling! (apparently not only did he not remember that letting the bolt go forward to chamber a round also fired the gun, but pulling the Uzi's trigger (necessary to allow the bolt to go forward) didn't tip him off!)
Those of us who actually understand how the Uzi works found the "officialese" of the accident report amusing, in that it described the Uzi as "a select fire, magazine fed weapon with a manual safety and a grip safety that positively preclude the possibility of an accidental discharge...see note A..."
Note A was not attached to the copy of the report I read but we all figured that it had to say something like "positively preclude the possibility of an accidental discharge, until you give it to an idiot!"
Hope this "simple" explanation helps your understanding and answers your question about why SMGs don't have another "safety notch".
Feel free to ask about any other questions you have. We're here to answer to the best of our ability and experience.