"How could Substack improve this situation for technical writers?"

How about this: don't run a dumb as rocks Web Application Firewall on an endpoint where people are editing articles that could be about any topic, including discussing the kind of strings that might trigger a dumb as rocks WAF.

This is like when forums about web development implement XSS filters that prevent their members from talking about XSS!

Learn to escape content properly instead.

Surprisingly simple solution

hire a cybersec person. I don't think they one.