I run a product similar to Mintlify.

We've made different product decisions than them. We don't support this, nor do we request access to codebases for Git sync. Both are security issues waiting to happen, no matter how much customers want them.

The reason people want it, though, is for SEO: whether it's true or outdated voodoo, almost everyone believes having their documentation on a subdomain hurts the parent domain. Google says it's not true, SEO experts say it is.

I wish Mintlify the best here – it's stressful to let customers down like this.

What makes you say that Google claims it's not true? Google claims subdomains are completely two different domains and you'll lose all the linking/page rank stuff according to their own docs regarding SEO. Some SEO gurus claim it's not so black and white but no one knows for sure. The data does show having docs on subdomain is more harmful to your SEO if you get linked to then a lot.

To my knowledge it's not as much hurting the parent domain as having two separate "worlds". Your docs which are likely to receive higher traffic will stop contributing any SEO juice to your main website.