GiffGaff's is enabled by default. I hate the way they (all) phrase it too, like you can disable it if you want to access 'bad stuff' - no, I just want not to be told what I can and can't access, 'no filter', one less place for anti-privacy logging potential etc.
EE's Content Lock is enabled by default, and is called out in the article itself. To opt out of Content Lock, you must undergo age verification, either via ID checks in person at the store, or via a credit card.
Please note: All new and existing accounts with Content Lock enabled have the "Moderate" setting applied by default. [1]
Archive.org is also available on my EE phone (when using mobile connection) - it's possibly I explicitly opted out of parental control filter, but I don't think I have. I'm less sure about that.