Denying access to financial sites due to outdated Safari Version

MacBook Air 13″, macOS 10.13

Software update may not be available for the macOS installed on your Mac.

Consider upgrading to a later version if compatible with your system.

Old versions of macOS may be compatible with your Mac.

Update: A later version of the currently installed software. Learn about macOS Big Sur updates, for example.

Upgrade: A major new version, often with new system requirements. Each macOS upgrade has a unique name, such as macOS Big Sur

 Learn more about installing the latest macOS upgrade. Some old versions of macOS are also available.

Published Date: June 11, 2021

    https://support.apple.com/en-us/HT201541

i think I can explain what is happening and why. Safari is installed as part of the operating system and it normally it cannot be individually upgraded. It is periodically updated when the computer's operating system is updated or upgraded. This is not an unusual situation. The reason is that web site engineers write their software to work with specific browsers and specific versions of them. You can hope that your web site's developers eventually update their site to work with Safari, or you can use another browser that does work such as Firefox or Chrome. They can be individually updated as needed. Finally, even if you are able to update Safari by upgrading your operating system, there is no guarantee that it will be compatible.