Process to block specific website in Safari app on iPad and iPhone changed recently with the released of iOS 12 under new Screen Time feature. Follow below steps to block specific website in Safari app on iPad and iPhone with iOS 12.
Step 1: Go to Settings > Screen Time > Content Privacy & Restrictions.
Step 2: Toggle Content & Privacy Restrictions to on and tap Content Restrictions to proceed.
Step 3: Tap the Web Content.
Step 4: Enable Limit Adult Websites option.
Step 5: Tap Add Website at the bottom of the screen to enter the URL of the website you like to block.
Step 6: Tap Done after added in the website URL.
Step 7: If you need to add more websites, return to the same place and keep adding more website URL into the list.
Apple Support: Use parental controls on your child’s iPhone, iPad, and iPod touch: Prevent web content