Any lock which can keep thieves and prying eyes away can also bar the rightful owner when the key is stolen, and this fact has left doors unlocked everywhere Klefki are found. Whatever shape of key is used, Klefki inevitably steal them, and therefore the practice of using locks has been slow to catch on in much of the world. This relucantance was somewhat out of fear of Klefki, but also came from cultural norms which relied more on alerting neighbors than simple locks in securing possessions - customs which evolved when Klefki had a far greater range than today.
A few individuals in every era have captured Klefki of their own, reasoning that even if their keys were stolen it mattered little if they knew where to find the thief. Klefki's loose definition of "key" has led this strategy to typically backfire, for the Klefki will steal anything which unlocks a door even if they must heavily modify the item to fit it on their ring.
Legendary pokemon have often been sealed away in this manner – either by the pokemon themselves, or those who feared them – and many ancient treasures remain likewise barred, awaiting the right Klefki to stumble past them. Treasure hunters have been known to catch every Klefki they find and try them everywhere sufficiently storied and conceivable, and this brute force strategy has surprised many with its successes; Klefki rings often include ancient keys, for Klefki have a nearly endless lifespan and their keys are rarely removed.
In the modern era, those who truly require security have abandoned keys entirely, to mixed results. Criminal organizations have found a defense against Klefki, but only by giving passwords to every member who needs them, one of whom is inevitably foolish enough to blurt them out to the hero.