75% of users use only the browser, and maybe the downloads folder of their file explorer. OS basically doesn’t matter as long as they have that.

I think by official OP meant stuff like Infinity is a FOSS client and not the official reddit app instead of alternatives.

I personally found Nougat to be the best, and then they focuses more on UI than features. (They also stopped the naming after that)

I believe Pixiv is exactly what you describe for artists, but not open source.