It's worth noting that vitamin D has a lot of high-confidence but low-effect results going for it, including a lot of strong meta-analysis over many health outcomes. What I mean by this is that it usually seems to conclusively either do something beneficial, but minor, almost nothing in the case of many studies, and in the case of some others, something very beneficial (as we've seen a few of in relation to covid recently).

Although Vitamin D isn't a particularly exciting supplement most of the time, that isn't enough of a reason for us to ignore it, as the proper way to make the decision of taking it or not would be to perform a cost/benefit analysis (as we should with any other supplements, lifestyle changes, etc).

The costs of vitamin D are just about as low as any supplement gets, and can be between $0.01 and $0.05 per day. The non-financial costs such as side effects or potential adverse effects are practically non-existent as well. So I do think that even those that have a healthy bit of skepticism over some results should still consider supplementation - there are a lot of potential benefits even if some are minor, and almost no risks or real costs. The optimal way to supplement is to get a blood test before+after starting supplementation in order to be confident you're taking the correct amount, but this isn't required of course. But I think the cost/benefit analysis is clearly in favor, so I didn't hesitate to add Vitamin D to my list of supplements that I take daily (https://nearcyan.com/supplements/ if curious), along with many others at this point.