I tried to look it up but I wasn't very successful. What they do in their free tier keeps being a mystery to me. In the $20/month is the the core ruleset from ModSecurity. I don't need to pay them $20 to deploy that for me, the dataset is free and publicly available. I've just installed it on my VPS... It's only a few lines in Nginx to enable that.
And what you're talking about is $200 a month. I seriously doubt anyone here uses that plan for their homeserver. I wouldn't pay $2400 in a year for it.
I still don't get how that would work. Sure you can filter spam that way. And migitate attacks while the worst wave washes through the net. Or do machine learning and find out if usage patterns change. But how would it extend to 0-days faster than the software gets patched? This sounds more like snake-oil to me. If someone finds a way to inject something into a Nextcloud plugin and change things in the database so they have access... And then they do it to 100 cloudflare customers... How would Cloudflare know? If it's a 0-day, they -per definition- don't know in advance. And they're just WAF, they don't know if a user is authorized by mistake or if they're supposed to have access. And they don't know anything about my database, since it runs on my machine. And they also don't know about the endpoints of the software and which request is going to trigger a vulnerability unless this manifests in some obvious (to them) way. Like 100 machines immediately start blasting spam through their connection and there is one common request in the logfiles. Otherwise all they can do is protect against known exploits. Maybe race the software vendor and filter things before they got patched. I just don't see any substantial 0-day protection that extends to more than "keep your server up to date and don't use unmaintained software." Especially not for the home-user.