Valid strategies don't permanently stop working if they're based on price action as a manifestation of market behaviour. If market behaviour changes of course, such a strategy will either increase or decrease in effectiveness but it can't become invalid: plus the specific market changes will be visible in price charts.
Strategies which are not soundly based might well work for a time and fail for a time. Which can make them legitimate choices for some traders - use them when they work, switch to something else when they don't. And either way, blame the market.