Yup, it appears to be gone at present. Some of my previously purchased tones are still available but I tried to purchase one this morning and it didn't show up. All my previously purchased tones are available to use as ringtones. They cannot, however, be selected for anything else. No one here would know Apple's plans for the future. It's unclear if this is an intentional change or a byproduct of something else. I'm leaning toward the latter but, we'll see.
I suggest that you submit feedback to Apple:
Product Feedback - Apple
Note that, as of this morning, iOS 17 was not an option in the drop down menu. Just make sure to put it in the text box.