Yeah, but given the sue-happy nature of the American public I can see why they err on the side of caution.
And the Yosemite Rangers are nowhere near as overprotective as the Grand Canyon ones
FWIW, the namesake mist from Vernal Falls does tend to create 5-10 foot icicles above the upper portion of Mist Trail.
(Not to mention coating the stairs with solid ice). Just be careful if you are ignoring a closure warning.
The closed panorama cliffs portion of the JMT is also quite dangerous in the winter- as the retaining wall traps the snow and ice.
Crossing that was one of the most astronomically stupid things I've done in my life:
So the official "Recommended" winter route is to head up the JMT to Clark Point, dip back down to the top of Vernal Falls, and then head up to Nevada Falls from there.
All of this is pretty much a crapshoot in March- the trail could be completely dry or under 10 feet of snow depending on how wet the winter was and how warm the spring started.