Certificates generated after July 1st, 2019 by versions of mkcert prior to v1.4.0 will not work on macOS 10.15 Catalina and iOS 13. Please update mkcert and regenerate the affected certificates.

The root CA is unaffected and there is no need to rerun mkcert -install.

— @FiloSottile

Under MacOS Catalina Public Beta 2, after installing mkcert via Homebrew and running the root certificate installer, my mkcert generated certificates are rejected in Safari with the message 'Certificate is not standards compliant' and in Chrome with 'ERR_CERT_REVOKED'.