I don't know about LiveJournal, but I don't believe you can host any interactive content on substack (without hacking substack at least). You can't sign up and host a phishing site, for instance.

User-uploaded content (which does pose a risk) is all hosted on substackcdn.com.

The PSL is more for "anyone can host anything in a subdomain of any domain on this list" rather than "this domain contains user-generated content". If you're allowing people to host raw HTML and JS then the PSL is the right place to go, but if you're just offering a user post/comment section feature, you're probably better off getting an early alert if someone has managed to breach your security and hacked your system into hosting phishing.