The FLCCC has been pushing Ivermectin, but I am not aware of an open letter analogous to this one or the similar one for masks that is signed by 100+ doctors & scientists.

I have not personally read the Ivermectin literature, but I think the FLCCC's methodology is good. (Paul Marik of FLCCC is a signatory on this vitamin D letter, BTW.) But it's notable that no one has Ivermectin deficiency, whereas more than half of people have insufficient vitamin D, especially in winter, so increased D intake is justified even in the absence of this COVID19 pandemic.

Not an open letter - but a manuscript of 30ish studies - I assumed there were probably ~3 doctors on each study.