In reality that was the core change - putting it into the description of the monster.
In 3.5 immunities were extremely common and were frequently by monster category. So constructs were blanket immune to critical hits and precision damage because whoever wrote them was thinking of golems, meaning that a clockwork monsters (clearly constructs) were immune to getting parts of their clockwork broken.
4e removed these blanket immunities and only handed them out to individual monsters very stingily; fire elementals were immune to fire but most other monsters had either resistance or triggered effects. And whoever designed the monsters took an "if you're not sure then allow it" approach. So I think there's only one group of monsters immune to being knocked prone in the whole MM1 and it's not oozes (it's Gorgons (bull type not Medusa) if anyone was wondering)