Apparently the verb pleuvoir can only be conjugated in the third person, but I don't understand why you would ever conjugate it in the plural. If Il pleut means It's raining, what does Ils pleuvent mean?
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You could use this form in a figurative way, when describing objects falling down or being thrown in big quantities. It can also be found in sport comments, usually tennis and maybe boxing, for example Les coups droits pleuvaient sur le Central (litterally Forehands were raining on the Central Court).
More accurately (IMO) coups is a common word for this, but many other terms expressing something abstract can also be used: insultes or injures is a common exemple in dictionaries too. The TLFi has a bunch of examples if you care to look at it.