To exclude sending an email at send time to those with a record on the 'Exclude' Data Extension where the primary key is Subscriber Key, the developer should use the following Exclusion Script:
ampscript
Copy code
RowCount(LookupRows('Exclude', 'SubscriberKey', _SubscriberKey)) > 0
This script checks if there are any rows in the 'Exclude' Data Extension that match the Subscriber Key. If the count is greater than 0, the email is excluded from being sent.
References:
* AMPscript Guide
* Salesforce Marketing Cloud Documentation