This is such an important feature and it’s not implemented. Incredible.
To be specific, in Strapi 4.16.2 (the latest version at the time of writing this), adding the required: true to the relation attribute still does not prevent the creation of the entry in the interface, although the relation field has a red asterisk close to it, like normal required attributes have. This is strange because Strapi automatically updates the types once we do this. So, clearly, there’s a bug in Strapi, which has been around for so many versions and years, and the developers don’t seem to care. Moreover, in the interface, we cannot set this relation attribute to be required, like for normal attributes.