How do I whitelist a domain in iOS restrictions web filter?
According to Andreas, he recently tried it again and it works fine now. But here's a somehwhat comprehensive answer anyways:

There are two ways to whitelist a website:


From Safari: When you try to access a website and it gets blocked, tap "Allow Access" and then the restrictions passcode.
From Settings: Go to Settings > General > Restrictions > Enter the passcode. Then go to the Safari settings. There, add your website to "Always allow" list. In your case, adding "wikipedia.org" would be enough.


Unfortunately, there are no ways to add wildcards like "*.google.*", which is what I need and why I ended up here.

Note: If the website you are trying to whitelist is in the Never Allow list, then remove first from that list. Otherwise whatever you do, that website will still get blocked since the Never Allow list trumps the Always Allow list.

About Domains and Subdomains:
Domains trump subdomains. In other words, if apple.com is on the blacklist, all subdomains, like developer.apple.com for example, will also be blocked. 

On the other hand, if developer.apple.com is on the blacklist, but not apple.com, then only the former will be blocked and the latter would still be accessible.