Recommendation for easily breakable bike lock

I suspect your question is framed from the point of view of your experiences. Any weak lock that can be defeated by a tool in your pocket can be defeated by the same tool anyone else carries.

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Instead of wanting a lock that is weaker, you might consider a lock that is more robust, AND to perform periodic maintenance on the lock like you would with your bike. A routine oil does wonders to prevent crunchiness.

Consider buying a lock that has some way of shedding water, rather than collecting the water and rusting.

You might choose a lock with brass construction (brass doesn't rust) or perhaps stainless steel.

Another solution is to make sure the lock hangs in such a way that the keyhole is not facing upward.

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If you really want a weak lock, get a velcro cable tie. It provides no security other than against a snatch-and-run. But its reusable, soft on the frame and won't scratch paint. You can also clip your helmet's chin strap around the forks and through a wheel. Security offered is basically zero.