This option has always been available for the same constituents endpoint.

It would be much more efficient to get a list of relevant gift custom fields instead of every custom field which we then need to filter.

This is the gift custom field list endpoint I'm referring to: https://api.sky.blackbaud.com/gift/v1/gifts/customfields

This is the equivalent constituent endpoint which already has the option: https://api.sky.blackbaud.com/constituent/v1/constituents/customfields