@flowerchild2000 Probably a rather small percentage. A great way to check could be looking up each domain's authority and backlinks but SEO APIs are expensive :/
I spent the last 2 days scraping Reddit to collect a total of 18167 unique domains of promoted startups and other websites. The data comes from r/SideProject and r/SaaS as they seem to be the most common places to self-promote.
It's not all exactly "startups", but about 95% of the links are...