Twenty now lets admins set record validation rules. A rule is a simple formula that Twenty checks every time someone creates a record. The post gives three examples: a website for every company, no personal emails for contacts, and a source on every deal over $10k.
If a new record fails a rule, the user sees a clear message right under the field. They fix the value and save the record again.
The post does not say which plans include the feature or when it ships.