Is the Samsung Galaxy S9 encrypted by default or can it be encrypted? Basically, I travel a lot with my phone and having it encrypted would be very useful in knowing that a crook wouldn't be able to gain access to my data should my phone be stolen.

I heard from one source stating that all devices running Android 6.0+ must be encrypted by default in order to be certified. 

However, I also heard from a Samsung representative that they removed the option to encrypt the internal storage after Android 7.0.

What's the actual case?
I believe you are seeing correct sources on both accounts. 

Android 6.0 made it compulsory if you are using Google Play Protect, this will show Certified Device in Google Play, You open the Play Store and go to settings, then scroll to the bottom, it will show you the certified status - therefore the encryption status.  

It makes sense to remove the obsolete option past that point.. Android 7.0.  

It may have been an unintentional oversight or the developers still required the option to automatically return true.