They said it would be with us in 2016 but... We Are in 2017 and still no news
2016
"We are glad to confirm that the Mazda brand has joined Assetto Corsa and will be represented by some of its most iconic models: the first-generation Miata and the brutal and awesome 787B will be added to Assetto Corsa as free updates later this year"
Source
http://www.assettocorsa.net/assetto-corsa-v1-5-dev-diary-part-33/
2016
"We are glad to confirm that the Mazda brand has joined Assetto Corsa and will be represented by some of its most iconic models: the first-generation Miata and the brutal and awesome 787B will be added to Assetto Corsa as free updates later this year"
Source
http://www.assettocorsa.net/assetto-corsa-v1-5-dev-diary-part-33/