Google launched the very first iteration of the Android M developer preview back during Google IO at the end of May, and just like last year with the Android L Developer preview, Google mentioned that at least one more version of the Android M Dev preview would be making its way to the compatible devices which have already been able to get their hands on the first build. Just earlier this afternoon the second build of the Developer Preview for Android M was released, giving eager users a chance to get their hands on some of the new features coming to Android M when it gets an official and final build launch later this year.
While there are due to be some more features uncovered between now and the few days, one that is already being noticed is the new System UI Tuner, which allows users to tweak or even remove some of the status bar icons. This has long been a customization hound's dream to have the power and capability to mess with the status bar icons without having to do little more than venture into the stock settings menus, and with Android M it appears that's going to be possible, at least, assuming Google decides to keep this feature as an addition to the final build of Android M when it releases. There is a perfectly good chance it will stay, but as always with things like this, there is also a chance that Google could leave it out since nothing is final till the software is pushed out.
You can enable the System UI Tuner from the developer options menu, and once it's turned on you should be able to see the System UI Tuner the main settings menu. As far as status bar options goes, you can turn off icons for Bluetooth, WiFi, do not disturb, ethernet, and many more options, but the System UI Tuner also lets you customize and tweak the quick settings, and there's an option to "show embedded battery percentage," which has been a feature among third-party home launchers and OEM custom UI skins for quite some time. There's also a demo mode which is likely there for developers to take advantage of. In any case, if you have the Dev Preview 2 of Android M installed, you can check these out now.