The captcha page, sure, maybe. I can't think off the top of my head what would happen on that page that wouldn't be related to Cloudflare/reCaptcha. I yielded that is a decent feature request. But plenty of actual interstitial pages served by Cloudflare aren't necessarily caused by Cloudflare. Like the fact you get a Captcha at all isn't Cloudflares choice most of the time, it's the site owners. And having [email protected] on that page would 100% cause people to write in saying they don't want to see captchas. That's not the appropriate party to reach out to requesting to stop seeing captchas for a specific site. Now, SOMETIMES it's an automated incident because of your IP, so then you DO want to reach out to Cloudflare.
Same with 500 series errors. Sometimes it's the website not responding, but sometimes it's Cloudflare not interacting properly.
So yeah, I think the truth of the matter is in the middle here. In terms of priorities, I have no doubt this is pretty low on their list. Why would it be any higher when they serve the technical purpose they were created for? The rest of that is QoL with minimal impact on customers compared to many other issues that go wrong with the network that have considerable impact on customers and visitors.
"many of these errors are delivered at request of the site owner" For those, put the site owner's contact method there. Even a physical mailing address, fine by me, I'll send a letter (something a spammer would not do) if it's important enough to me to do so.
"or related to the site not working (404s, 503s" those pages don't deliver a Google CAPTCHA or don't say "You have been blocked". If they can determine whether a page should have a captcha and/or that text, then that if statement can also include showing contact info.