Hi there ajahn
macOS High Sierra last received an update from Apple over four years ago, on July 9th, 2018, and is considered end-of-life. Typically, although they don't give an official date for this, Apple tends to provide security updates for the three most recent major versions – in this case, macOS Monterey, macOS Big Sur and macOS Catalina.
When macOS Ventura is released (probably within the next few weeks, as of this post, although unconfirmed), macOS Catalina will probably not receive any more security updates and will also be considered end-of-life.
As a security-focused company making a security-focused product, we want to make sure that any new major version of 1Password we produce runs on a currently-supported version of a device's operating system that is still receiving security updates. It's the same reason that we require Windows 10 or 11 to run 1Password 8 for Windows, for example, or iOS 15 or later to run 1Password for iOS. (iOS 15, despite not being the latest version, received its most recent update, 15.7, on September 12th, 2022.)
I hope that explains our rationale, but please do let me know if you have any questions or would like any further help. :)
— Grey
