The Android O battery percentage indicator now sits next to the battery icon instead of inside of it, a minor change that likely won't bother some but may be a bit of an annoyance for others. If you never used the battery percentage indicator option then you likely won't be affected by this change at all, but for those that did use it this was a nice way to not only catch a glimpse of your battery life by looking at the icon, but it made it so you really didn't have to guess at how much battery life you had left, and you didn't have to enter into the battery settings menu to find out what the percentage was.
While this isn't disappearing, the fact with it now being placed outside of the battery icon in the status bar just to the right of it, instead of sitting inside of the icon, this means that it will be taking up that much more space in the status bar. Not a lot of space, but just enough that it will have to displace a different notification icon that would have otherwise had room. Again this is minor, but it's a change that could very well matter to some users as they like to make the most of the space for notifications.
The percentage for battery life toggle can still be found in the System UI Tuner just like before, so there shouldn't be any worry about not being able to find it as Google hasn't moved it to a new location. One thing that is important to keep in mind is that even though this is how things are in the first Developer Preview, there is nothing stopping Google from reverting the battery percentage back to being inside the battery icon. There are three more Developer Previews left before Google releases the stable version of Android O to the Public sometime in the third quarter of this year, and nothing in Developer Preview software is final, which means that if you hate this change things may go back to the way they were. Alternatively, if you love the change then you may get your wish and Google could keep things as they are in this version.