I had same issue (same macOS/Safari versions).
I solved it for myself by installing USERTrust RSA Certification Authority SHA-2 root certificate from Sectigo website. It is in a knowledge base article called How to Download & Install Sectigo Intermediate Certificates - RSA.
Go to Sectigo website -> Support -> Knowledge Base, and it is the most viewed article as of this writing. The SHA-2 root certificate is towards the bottom of the article (just above the expired one).
After downloading, I opened the certificate in Keychain Access. It is untrusted by default, so I did right-click -> Get Info on the certificate, expanded the Trust part and chose When using this certificate: always trust. You need a macOS admin password to make the change. When I did it first time, my computer became unresponsive... (old 2008 MacBook), but after a reboot I could repeat the change and it went without a hitch.
I'm not a certificate expert so I don't promise it is the "right kind" of workaround, or it works for all websites. I just tried the one website which was failing me before, and I saw Safari is now using the newly installed SHA-2 root certificate for it.
Hope that helps.