What occurs if you change the datatype of a custom field set as an external ID to a type other than text, number, or email?

When a custom field is designated as an external ID in Salesforce, it serves as a unique identifier for records and can be used for data import or integration purposes. External IDs specifically require the data types of text, number, or email to function correctly. If the datatype of an external ID field is changed to a type that does not conform to these requirements, the functionality that allows it to act as an external ID is lost.

This is because external IDs must be able to uniquely identify records through a consistent and compatible format, which is not guaranteed if the datatype is altered to something outside the established categories. Consequently, once this change is made, the system no longer allows the field to serve its purpose as an external ID, effectively rendering it inoperable for that type of identification.