There seems to be a misunderstanding regarding the deprecation timeline. The /api/user endpoint itself is not being deprecated at the end of the year; only JWT authentication for that API is being deprecated. You can continue to use the /api/user endpoint by switching to API key-based authentication.
Steps to Continue Using /api/user:
Update your integration to authenticate API calls with an API key instead of JWT. Access data.salutation as usual through the /api/user endpoint. This data is part of the user.data object, which is populated by your integration and not automatically generated by FusionAuth.Steps to Use /oauth2/userinfo:
Write and install a UserInfo lambda which can read the user.data object and augment the userinfo response to include the data.salutation value. Docs on this lambda.