Note: These settings will block websites URLs on all browsers in your iOS device. It doesn't work on just Safari, browsers like Google Chrome also will be affected.

How To Restrict Specific Websites In iOS Device

This section shows you how to block specific websites on your iOS 11 downwards and iOS 12 upwards.
How To Block Specific Websites In iPhone/iPad Safari(iOS 11 down)
This feature allows you to block a particular site from opening in your iPhone's safari browser.
Step 1:
Open your Settings; From your app tray, locate and open your Settings app.
Step 2:
Tap on General then Select the "Restrictions" option.
Step 3:
Tap on "Enable Restrictions".(if your restrictions were Enabled already, you'll not see this option - jump to step 6)
Step 4:
Create a 4-digit password that your kid or whoever wouldn't be able to guess.
Step 5:
Re-type the 4-digits password to confirm it. (Don't misplace this passcode)
Step 6:
Scroll down and locate the "Websites" option, select this.
Step 7:
In the "Never Allow" section, select "Add a Website". This option is usually the last on the list.
Step 8:
Type the website URL you wish to block. Most websites have different versions for mobile phones and desktop computers, your iPhone will take into consideration the websites you type here.
Open the site on both a mobile and pc browser, confirm if the URLs on both browsers are similar, if they aren't similar, get both versions and block them separately.
Step 9: Tap on "Done" to implement your settings.
Repeat the above process for every site you wish to block. If you notice that a particular site keeps getting through, visit the site on a mobile and pc browser and get both URL version and paste it in Restrictions.

How To Prevent Specific Websites On iOS 12

The steps are a little bit different.
Step 1:
Open your settings app from your app tray or app menu.
Step 2:
Tap on "Screen Time".
Step 3:
Select " Content & Privacy Restrictions".
Step 4:
Select "Content Restrictions".
Step 5:
From the second section, tap on "Web Contents".
Step 6:
From the "Never Allow" section, select "Add A Website"
Step 7:
Type the URL of the website you wish to block in the text field provided.
Step 8:
Tap on "Done" to confirm your settings.
Source from How-To-9ja » How To Do It Yourself (https://howto9ja.com)

See how to allow specific sites, block adult contents etc here https://howto9ja.com/how-to-block-websites-on-iphone-ios-11-12-etc