In early betas of macOS Catalina, the iTunes Store was visible, but in recent betas it did not show up in the sidebar of the Music app if the user was signed into Apple Music. That seems to be the default now: if a user has an Apple Music account, they won’t see the iTunes Store. You can display it, if you wish, in the Music app’s Preferences, on the General pane, but if you’re a streamer, you won’t see it by default.
This seems like a graceful way to handle the virtually-complete transition of listeners from purchasers to streamers. For those of us who do both, it’s a preference change. Pretty straightforward.
What this means for the future of the iTunes Store seems obvious, but it is not a future I’m willing or eager to accept.