Comment by yorwba
https://phoible.org/parameters has the data you seek: 5% of languages in the database have eth (ð) and 4% have theta (θ). Z is not a 'th' sound and fairly common at 30% of languages, though.
https://phoible.org/parameters has the data you seek: 5% of languages in the database have eth (ð) and 4% have theta (θ). Z is not a 'th' sound and fairly common at 30% of languages, though.