When typing an address in Safari, the period (full stop) key hides some alternative options. If you hold your finger on it for a second, a range of domains (such as .co.uk, .org, etc.) will appear. Slide a finger to one and release. This only works when typing web or email addresses, not when typing text in apps such as Notes or Messages.
Keyboard shortcuts: type web addresses quicker | iOS 11 Guide
- by Tom Rolfe