It looks like with QtWebEngine 5.15.2, OCSP is always used, even if setUseForGlobalCertificateVerification was never called on any profile.

This can be observed by running e.g. Wireshark (or tcpdump, though I haven't tried), filtering for ocsp and running simplebrowser.

Alternatively, drop outgoing HTTP packets entirely with something like sudo iptables -A OUTPUT -p tcp --dport 80 -j DROP and you will see HTTPS connections failing.