Disclaimer, the following is my observation so I’m not sure how generalisable it is.
WoD and onward started having rare spawns up at all times, and dropped 100% of the time. But some rares did not always appear. This stayed in legion, for the most part. And in Shadowlands, rare spawns that cna be encountered during leveling drop 100%, but everything else (cosmetics, mounts, toys etc) is rng, and they don’t always appear.
Before WoD, rare mobs were rare but i believe always dropped items.