The Message 'This applicant can be hired only through Onboarding' is not a 2023 release feature or a new applicant workbench feature. It is an old validation mechanism added last 2022.
The message essentially shows when you have both Onboarding and MPH enabled. You can hire the candidate either via Onboarding or via Manage Pending Hire, and the system is telling you that this application meets the criteria for your onboarding filter, hence it should pass through onboarding.
3195224 - Warning on the candidate's job application: "This applicant can be hired only through Onboarding." - Recruiting Management
Validations on Applicant Workbench
It cannot be hidden technically, but you can rephrase the message through Manage Languages.
2576546 - Using the Manage Language Feature to Change Labels in SuccessFactors
We ran into this last year when implementing RCM/ONB for a new country. You can find more info in this KBA and the message can be addressed through Manage Languages so end users don't see it.