This appears to be an inconsistency (and that you would instead search email through person or custom field). I’m speaking to the team in charge of this endpoint to gain some clarity.
Until this s fixed/looked into further, you can assume that you can search based on an Item’s default Fields. For Leads, the only default Field is Title
For further clarity, email , organization_name , person_name , phone are legacy fields so if you have old Leads that were added, they could still be searched by these Fields, but no new Leads would match.
We’ll update the Documentation to reflect this. Thanks for bringing this up