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I don't think it is slang. I think it is a tad old fashioned, but any native speaker would understand it.

I wouldn't call it slang, it is the common way to refer to the act in media, and the government uses it, e.g: https://www.gov.uk/report-flytipping (they do define it, however, as you say, it would be unclear if you aren't familiar).

I don’t think Aus/NZ would use it in this manner. It would be more likely to see “no dumping”.

I remember going on a walk in London with my dad when I was little and we ended up in Hyde Park (this is late 1970s). There was a sign that said 'no dumping" which he chuckled at and explained to me the slang term 'dump', as in 'to defecate'.