Starting with version 15.1, the iWork suite is now part of the Creator Studio bundle.
The apps were switched to a freemium model, with old apps not receiving updates anymore, and new apps being full of Creator Studio subscription upsells across the menu bar options and other surfaces.
Personally
Besides the "Premium" stock imagery, the rest Creator Studio-paywalled features in iWork are just GenAI slop. From the Creator Studio bundle, I interacted only with the iWork suite, which was free with no upsells, and Pixelmator Pro, for which I bought a perpetual license. I have no interest in Final Cut, Logic Pro, Motion, or other apps from the suite, so I can't justify paying $129/year for it.
On the Apple Support website there's a mention that there is an MDM configuration option to suppress these prompts for corporate devices.
Thus, I've made a .mobileconfig file that applies this setting to Keynote/Pages/Numbers.
From my testing, it works and removes all most annoying instances. I only noticed one leftover in the app menu.