We are also looking for the same, and this feature request is 7 years old without any progress. We have a self-hosted SSO system built using KeyCloak to centralize all our internal infrastructure logins for staffs, and WHMCS isn't supporting it still. Is there any chance you could implement OAuth support for the Administrator staff logins too using the most common providers including KeyCloak OpenID Connect?

I hope this will include in the release in the near future.