I’ve tried using Screen Time in the past to block some gaming websites on my daughter’s iPhone. It worked okay at first. But I noticed that sometimes the sites could still be accessed through Chrome or other browsers. What's more, Screen Time can be pretty easy for kids to bypass.
Then I discovered a dedicated productivity and parental control app called Cisdem AppCrypt. It’s password-protected, so only I can change the settings. (if you want to block the sites for yourself, ask your friend or a family member to set the password for you).
It can block any specific websites/webpages on iPhone permanently and is super easy to use. Also, it's free. If Screen Time isn’t working for you, I recommend giving it a try.