pop up screen keeps saying 1Password update required - Legit?

You don't mention or tag what platform you are using, but this looks very suspicious. Is this appearing on your desktop or in your web broswer?

In any case on any reliable platform, you can and should use your trusted software management tools to check for updates to installed software. For example, in Chromium based web browsers you should use Extensions to manage and update extensions.
Platforms on which software often implements its own update process - checking, schedule, notifications, interface, etc. - present a serious problem to the user. Nevertheless you can and should check the current version of 1Password in Settings > About, then compare that to the current release for your platform.

Please follow up with your findings so that others who encounter the same thing and end up here can learn the result.

Reeseter wrote:

I'm on a Mac.
Not within the browser
Version 6.8.9 (689000)

How does that compare with the stable version of the application?

While you are checking and comparing versions, you should do the same for the browser plugin.