Is it still possible to purchase a monthly subscription to the Muse app (not entire CC)?
No, Muse is "End of Life". You can't purchase (you never could) or subscribe it individually. You only can download it as part of a complete CC subscription.
Thanks Gunter. As a longtime Muse user I am aware that it could never be purchased. Before, and for a time after, the EOL announcement Muse could be subscribed individually, just as can most other Adobe apps. I thought perhaps Adobe had become slightly more user-reponsive but alas!