Heatspreaders on RAM are actually very, VERY commonplace since DDR1, particularly for the faster varieties picked in gaming PC builds.
It's widely agreed on that they're more for overall aesthetic nowadays than any practical advantage in cooling, at least for standard unbuffered DIMMs. People love cases with transparent side panels to show off their hardware, after all.
Those heatsinks on the aforementioned Mac Pro FB-DIMMs, on the other hand? Nuts! Those might actually justify getting waterblocked instead, and yes, RAM waterblocks do exist. Some people just gotta liquid-cool all the things.