Jumping in here to provide a clear tech perspective.
Jagex do not store passwords in plaintext. Customer Support, developers and everyone else at Jagex do not have access to your passwords. Access to anything relating to passwords is controlled in line with GDPR, industry standards and common sense. Passwords are secured and protected using modern algorithms and all other functionality that is deemed appropriate by ourselves and the industry.
Seperately no capital letters etc. isn't great, but it is not related in any way nor representative of our security systems currently in place. Which is a common assumption I see made.
Our database of passwords / personal information have also never been leaked / compromised, to my knowledge. If it was, I believe we'd be legally obligated to reveal that information.