Armbian do a great job of handling support for a whole host of boards (including most I included in this list), so you'll usually have Debian/Ubuntu-based flavours. Vendor kernels and vendor supplied images will be hit and miss. Mainline Linux support is a flag you filter by on the benchmark comparison site linked in the article, but it's a difficult one to keep up to date and define exactly. It could have some kind of support, but miss out on display functionality, or WiFi yada yada. What would we then class as having mainline support? All hardware etc functioning? If so, very, very few will meet that definition.
I get the desire for the information, and perhaps I should have envisioned these types of questions, but all I initially meant for the post to be was a recap for people following me to see which boards I'd tested that were released last year :D