You can reenable the web interface in iCloud settings under system preferences on a Mac (not sure if it’s on iOS). The web interface will require just in time verification with an iCloud device to confirm log ins though. Works pretty well.
However I just found out Apple added a way to manage rules in iCloud Mail to iOS 17. It's in Settings > [Your Name] > iCloud > iCloud Mail > iCloud Mail Rules. They did not mention this anywhere, but it's there and it's new!
Unfortunately still unusable for me because it allows only a single condition. No AND or OR conditions. For example still not possible to filter by FROM and Subject and the same time. And you can only filter a very limited amount of fields: FROM, TO, FROM or TO, CC, Subject and List-ID. For example it's not possible to search the body.
iCloud Mail and Calendar are way too simple to be useful for me. Calendar only searches 1 year back, even if everything is synced (I actually need this quite often). No weekly view in portrait mode (I use this all the time in Google Calendar). Calendar and Reminders don't integrate at all (Google Calendar shows tasks from Google Tasks). Not possible to sync calendar subscriptions across devices.