この回答ではISBLANKが使えたと言っているけど、本当かな?
Long Text Area(32768)
ISBLANK (Long_text__c)
Error: You referenced an unsupported field type called "Long Text Area" using the following field: Long_text__c
LEN(Long_text__c) > 0
Error: You referenced an unsupported field type called "Long Text Area" using the following field: Long_text__c
Long Text Area(500)
ISBLANK (Long_text__c)
Error: You referenced an unsupported field type called "Long Text Area" using the following field: Long_text__c
LEN(Long_text__c) > 0
Error: You referenced an unsupported field type called "Long Text Area" using the following field: Long_text__c
Text Area(255)
問題ない
Rich Text Area(256)
ISBLANK (Long_text__c)
Error: You referenced an unsupported field type called "Rich Text Area" using the following field: Long_text__c
LEN(Long_text__c) > 0
Error: You referenced an unsupported field type called "Rich Text Area" using the following field: Long_text__c