Was this known early on about COVID? Early on in the lockdown I noticed that these vitamins were among the kind of weird items completely cleared out of stores. People were very selective... initially all FHD laptops and Chromebooks were gone, along with power strips, ibuprofen, hair clippers, etc.
The problem with that is there are a million reasons Vitamin D might correlate to better outcomes even if it's just a total co-incidence - generally healthy people might spend more time in the sun, health-conscious people might be more likely to take vitamins, other conditions may cause a lack of Vitamin D, etc. There's no proof that supplementing would actually improve outcomes.
This is the first study I've seen that claims to show directly that a supplement given during a hospital stay directly improved outcomes. It's a tiny study, but interesting and hopefully leads to more studies.