Do you have any source? I was initially suspicious of the same but found few content in reddit/other forums where multiple users said they have free plan and get millions of requests per day[1]. And I couldn't find a consistent reason for all the sites that cloudflare banned.
Cloudflare does not have any bandwidth limits or request limits in any way. They generally do monitor high usage users on lower plans to try and upsell them to enterprise but you dont ever have to actually upgrade.
The reddit post you linked seems to not really have any content anymore, but I would be willing to bet they broke TOS given you are only really supposed to serve html content over CDN when on lower level plans when you are not using the developer platform to host the content such as videos/images/etc.
Fwiw though, if you are a business you probably should at the very least upgrade to business for the SLA and to create support tickets if needed.
edit: figured out why the article doesnt exist the OP removed it because it was an issue on his end not Cloudflares. (Using bot fight mode on a route supposed to be used by bots) https://news.ycombinator.com/item?id=26072153
[1]: https://www.reddit.com/r/programming/comments/lfa1il/why_you...