No remedy yet. Apple has not moved any iBooks Author functionality to Pages 10.1 or later, but has permitted Pages to open iBooks Author (.iba) documents and retain the general appearance. You can read more about what that translation includes in Import your iBooks Author book into Pages for Mac.
Only the Pages product team, who do not participate here, know the future of iBooks Author feature set migration to future Pages products, or their potential introduction timeframe.
Although Apple suggested that iBooks Author would only continue to work on macOS Catalina, I have found that it works on all releases of Big Sur through 11.6.3, and continues to launch on macOS Monterey 12.2.1. There is no assurance however, that it will continue to work on future versions of Monterey.