That is a useful feature to have. Samsung removed that functionality from their messaging app, but there are still several other third party messaging apps available that still provide that functionality if you want to use one of those instead
That feature was actually removed from the native Contacts app on the S8+ devices after the update. So using a third party messaging app may have the feature but it will not be available in the native Contacts app. Which mean the user will add custom ringtones to the contacts in the Contacts app then go to the third party messaging app to add customer text message tones to each of those contacts. This is double work for a feature that originally came with the phone (which is not cheap); the same feature is also available in previous OS. I do not foresee Samsung or VZW fixing this issue no time soon and I would venture to say that all future Samsung phones will not include the text messaging tone field.
Have you considered this was a change made by Google and Samsung finally incorporated it into their builds?
I'm most definitely NOT a VZW employee. If a post answered your question, please mark it as the answer.