Browser stack did not had the betas or the just released Safari versions, we had to depend on someone in our team risking upgrading to latest OSX to be able to debug with just released Safari that broke WebGL on desktop too.
We do not target mobile users, we are fixing any bugs reported for Safari that are our code , but we only daily test with Firefox and Chrome because the developers and testers are using Windows or Linux(the team is in Europe)
i.e.
https://www.browserstack.com/test-on-iphone
I'm just trying to find the one I use to test my emoji stuff, since Safari is the only one that displays emoji domains correctly.