When typing an address in Safari, the . 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 in web or email addresses, not when typing text in apps such as Notes or Messages.
Behind the dot: secret Safari typing shortcuts | iOS 11 Guide [iPad]
- by Tom Rolfe