Depends on the system. This would be my second to last choice as an engineer. First choice should always be design the system so you can't get hurt in the first place - but nobody has any ideas on how to do that to a table say (or we have ideas but it no longer can do the job of a table saw and so must reject them). Second is to put guards in places - we have been doing that since at least the 1980s (probably before, but I'm not old enough to remember), but guards are not perfect and so people still can lose a finger even with guards used correctly (cheap guards often limit the functionality of the table saw by enough that everyone just removes them, but even good guards are not perfect). Only after the above would I look at stopping the system when a problem is detected. Last, but only if all of the above fails - is you put warning stickers on.

Let me emphasize: you should run the above list in order. If you can design a problem out then you are not allowed to put guards, brakes, or warning stickers on./

Most industrial machinery is designed with the above process. there is a lot of machinery from early days still around with out safety, but most industry has been adding guards and brakes to those were possible and replacing (machines from the 1950s are probably worn out anyway) the old stuff. Industry also has extensive safety training for the dangers they they cannot prevent other ways. The safety results for industry is much better than it was 100 years ago. Not perfect by any means, but much better and getting better [I was going to write every year, but random chance means some years there are more accidents than others despite the safety situation overall improving yearly]