This blog post points to the no longer available FAQ from wizards about prone oozes in 4e.
A few days ago, someone asked what was the difference between 4e and 5e in an internet forum . I tried to explain that I liked 5e over 4e ...
methodsetmadness.blogspot.com
4e FAQ
Can a gelatinous cube be knocked prone? In situations like this, DMs are encouraged to change the flavor of what is happening without changing the actual rules governing the situation. For example, the ooze could be so disoriented by the blow that it suffers the same disadvantages as if it had been knocked prone until it spends a move action to stand up effectively shaking off the condition
And yes, I point to a reference that points to a no longer existing reference. I know. It doesn't exist in the downloadable FAQ as far as I can tell, but that doesn't look complete anyway.
So there is another reference from rpg.net.
A giant constrictor snake is not immune to the "prone" condition. Let's say a Battle Master uses whatever maneuver knocks you prone because he wants everybody else to get advantage on their attacks. As a DM, do you say, "no, you can't knock a snake prone. That's just common sense. The Monster...
forum.rpg.net
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Oh and finally here is a reference to that FAQ entry here at ENWorld from 2008.