[diggan]

> How could anything be enforced on user-controlled servers?

New laws comes to mind. If a government decides to try to outlaw encryption again, cloud/hosting companies located there wouldn't have a choice but to comply, or give up on the business. The laws could also be made in such way that individuals are responsible for avoiding it, even self-hosters, and if people are using it anyways, be legally held responsible for the potential harms of it.