All the modular click on mount battery powered lights have a button to switch them off. Sometims you have to "scroll" through X blinking modes to reach it, but it's off, immediate light out.
A dynamo powered one, model with standlight (apparently they don't all have this), some/alot (I found brands that do provide it) scrapped this switchoff button, which has nothing to do with safety since it's the rider deciding on/off and leaving on means staying on till electrical load storage empty. The only reason left is to save some bucks, ie the price of a button and related provisions.
So for those that want to be able to turn a light off, just skip the brands that cut costs.