Your assumption that the mac is blocking this could be incorrect. It also could be a VPN software like Cisco AnyConnect which uses OpenDNS servers to ignore the local DNS servers and settings.
By the screen shot, Your mac is trusting the router 192.168.1.1 to filter all your traffic, and that router is the item that’s configured to filter some sites.
If you can’t find some network software that installs a kernel extension or uses VPN hooks / network filtering in Catalina, the next step is to look at the group that runs your network setup? You will need to discuss with them how they’re configured the filters that direct traffic.
Lucky for you, they left a calling cart there in the block page - email@example.com
Perhaps they have a log in process to ensure you’re not a minor. Think of this like mailing a package. If your mail service goes through a company that checks each address and blocks delivery to some, you would need to either get them to change their rules for your packages or choose another company.
In net-neutrality terms - you need a VPN or different agreement or a different DNS provider.
If this filtering is likely legally mandated in your country, check with your lawyer before you try to get around a content filter block for minors. We are not lawyers and can’t be your lawyer, but this is a very topical subject about freedom, censorship and the internet. Good on you for learning about how your privacy works. If you’re not a minor - vote for net neutrality and better transparency from your IT setup team. If you are a minor, get ready to vote and this is what algorithms and surveillance looks like in the 21st century.