Older versions of iPhone OS aren't new enough to have modern certificate authorities trusted, meaning only sites using older certificate authorities are able to have a secure connection.
Certificates:
👉ISRG Root X1 CA (+Let's Encrypt)👈
👉ISRG Root X2 CA👈
👉Signed iOS Bundle (iOS 5+)👈
certificates hosted by Updated Root CA's by Hanabi
Click to add Directly to Cydia