On fields of type Single-line string, Multi-paragraph text, and Structured Text, we used to allow both null value and empty strings (""), but in both cases the API would return an empty string.
The exists filter, which selects values different from null, would then return empty strings instead of null values. This behavior is confusing and unpredictable, so we decided to deprecate the exists filter for these types of field.
We created a new filter, called isPresent, that selects values that are neither null nor empty strings. It behaves as the opposite of isBlank. The exists filter will continue to work, but is now deprecated, and will be removed in future versions of our API.