Apple released released Safari Technology Preview 44 (Safari TP44) on Wednesday. This release enables Payment Requests by default, and it includes 38 other improvements and bug fixes, too.
Bug fixes and other improvements cover Conic Gradients, Image Bitmaps, Web API, CSS, Rendering, a bunch of Wen Inspector issues, Web Driver, Media, CSS Grid, Security, and Extensions.
Developers (or others wanting to living on the browser bleeding edge) can download Safari TP44 from the WebKit.org website.
Bryan, what is Payment by Default?