I used to access the motoko base libs at https://internetcomputer.org/docs/current/references/motoko-ref/iter/ or https://internetcomputer.org/docs/base-libraries/iter, but that’s now a 404.
It seems that they are now at https://internetcomputer.org/docs/current/motoko/main/base/iter.
Not sure when that changed, but may I kindly suggest to not break URLs so easily? They are in bookmarks, browser history and (worst) linked from documentation, possibly even in blackholed canisters.
If you have to change URLs, please leave redirects in place.