Test mode has been deprecated. It was originally designed to allow test card charges. While the intentions were good, the side-effects were just too much. For example, if you had created a couple of subscription plans in test mode, you'd need to re-create those plans again once switching to live. This would have resulted in a lot of duplicate work. Additionally, there were some bugs with Stripe Connect and the way that they handle redirects.
Test/Live Status dropdown has been removed (see above)