Passwords saved in your keychain are no longer accessed from device settings. For iOS/iPadOS18, saved Passwords and account credentials are now accessed using the new Apple Passwords App; when you updated to iOS/iPadOS18, this new App will have been automatically installed on your device.
This support page may be of interest:
Use the Passwords app to create, manage, and share passwords and passkeys across Apple devices - Apple Support
The iPad User Guide is a rich (and often overlooked) source of information about your iPad and iPadOS. Here is direct link to the relevant section of the Guide:
View your passwords and related information on iPad - Apple Support
The Tips App, installed by default on all new models of iPad, provides a brief introduction - and leads you to the User Guide. In case you missed it, here is a link to the Tips App:
https://apps.apple.com/app/tips/id1069509450