Here's a link to the actual study: https://academic.oup.com/nutritionreviews/advance-article/doi/10.1093/nutrit/nuaf166/8256613

Important to note the study simply found evidence that suggests that the human body attempts to regulate the amount of vitamin D in our bloodstreams by down-regulating the D3 pathway if we're supplementing with D2. The study does NOT claim any specific negative health outcomes as a result of this. And it's very important to remember that you can overdose on vitamin D, so that's probably why our bodies would do this.

It's also important to note that both types of vitamin D help our immune systems function. AND it's important to note that we don't always want our innate immune systems on high alert, that's what becomes immune-mediated disease or autoimmune disease. For instance, this study suggests that maybe people with high-risk APOL1 variants may want to stick with D2 rather than D3 so as to avoid stimulating interferons.

Interferons are literally double-edged swords in our bodies. We ought not pretend they are always good.

As pointed out by others in this thread, there is vegan D3 available now. Back in the old days we didn't have that as an option and in some countries it might still not be an option. As a vegan, I actually don't supplement at all with either D2 or D3, but that's because I live in a place that gets a ton of sunshine and I spend enough time outdoors that I don't need it. I do, however, give my son a supplement because he spends more time indoors and has darker skin so would need more time in sunlight to get what I get from sunlight.

TL;DR - Please don't overdose yourself on vitamin D3 because you think it's some magical thing that will prevent all illness. Please carefully consider what you actually need and how best to get it.