To that end, it was added to NSS 3.26/Firefox 50 (November 2016) and to Chrome 57 (March 2017).
On iOS, it was first added in iOS 10 (2016).
The root itself was created in June 2015.
Edit: they also have a list on their website - https://letsencrypt.org/docs/certificate-compatibility/#know... . It lists Android < 2.3.6 as being incompatible .. I wonder if that's updated for the non-cross-signed intermediates.