This is a subject that is always tinged by the desire of some to exaggerate their prowess and others understate their low standards. It seems to be 'the thing to do to show you are a real RF street shooter.' This is not directed against those in this thread, but against what I feel is a subject often distorted by tall tales or at least a lack of objective truth telling. Did I tell you how big that fish was?
While you can hand hold at very low speeds, some or more of the following usually apply:
You can't do it every time, so it is not reliable. I like reliable when special things unfold in front of my camera.
The results are invariably less sharp than a faster speed.
It depends on how large you print
It depends on your standards.
It depends on the subject suitability.
I believe that while shooting at very low speeds can be useful, it is far less useful than some might think. If I need to be sure I get the shot, I will shoot with faster film. I'd rather have more grain and a sharp shot. And people move.
Certainly the impression that many RF shooters can walk around gaily shooting unbraced 1/4-1/15 with their 50mm lens, or even a 28, and come back with a good quality roll of shots (unless they are shooting buildings while leaning against another) is not realistic, but reading some threads you'd think that you have a problem if you are not able to do this while riding a unicycle with a spiky seat. Invariably the images shown to prove this are frequently dull even when they are 'sharp enough.'
Here is something that many of you will laugh at, but I an confident holds true for most people shooting under real world rather than test conditions:
1/focal length is pretty good minimum for use under controlled conditions, unbraced, but still taking care not to pull your shots. Slower than this and you can get sharp shots, but less often the slower you go and even the best are not as sharp as at higher speeds once you dip two or more stops slower.
If you are working quickly and I mean taking snap shots, working under physically demanding conditions, or on edge, or excited, or scared, or working in the heat or real cold, or (insert anything else that can make it hard to hold steady) i.e. the real conditions under which many of the best street/docum/reportage photos are taken, you should be looking at twice as fast as 1/focal length to be sure you still get good frames even when concentrating. Minimum.
Some people are steadier than others, but when doing documentary work I do not dip below the above rule unless I am braced or am working sedately. I work at 1/FL when things are smoooooth. I work at twice this exposure (i.e. 1/15th with a 35mm) only when braced, or squatting, kneeling with shoulder against wall etc, only when everything has slowed to a very controlled pace and only then when I absolutely have to.
PS If using a camera that allows you to leave the film leader out (including Leica M by feel) then all it takes is to change to faster film not to risk blurry shots.