Besides user uploaded content it's pretty easy to accidentally destroy the reputation of your main domain with subdomains.

For example:

    1. Add a subdomain to test something out
    2. Complete your test and remove the subdomain from your site
    3. Forget to remove the DNS entry and now your A record points to an IP address

At this point if someone else on that hosting provider gets that IP address assigned, your subdomain is now hosting their content.

I had this happen to me once with PDF books being served through a subdomain on my site. Of course it's my mistake for not removing the A record (I forgot) but I'll never make that mistake again.

10 years of my domain having a good history may have gotten tainted in an unrepairable way. I don't get warnings visiting my site but traffic has slowly gotten worse over time since around that time, despite me posting more and more content. The correlation isn't guaranteed, especially with AI taking away so much traffic but it's something I do think about.