Alright, I withdraw that. Having done some homework, “beach” has been mainly used in meaning (1) both in real world and, as natural=beach, in OSM. While man-made shores like this may be called “beach” in real life… :

(Clovelly Beach, Sydney)

(Topla beach, Herceg Novi)

…we have an appropriate leisure=bathing_place for things like that.

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