(Updated 2020, still not solved)
It should be the case that anything in the settings portion of the iPhone is changeable by voice. It should also be the case that you can ask questions about what settings are possible and for various apps, especially Apple apps. I should NOT have to go to settings and navigate PAINFULLY through the various menus to find my settings. This is especially true when Apple moves these settings around from version to version. When Siri first came out this should have been done. But it wasn’t. Apple has added, “some” of voice access over time but not all. As a tech person, I many times need to answer other people's questions on how you can change various iPhone/iOS settings. This is sad. There aren’t that many, but the moment a non-tech person sees the settings app they freeze. It needs to be ALL voice modifiable.
Even for me, a tech person, it happens that I need to change settings but don’t know where to start. Recently, my iPhone decided to start substituting every time I typed “ok” with “Okay”. As I've turned off a lot of my auto-correction, because my acronyms are always changed on me, this was a little bit irritating. I put up with it for a while but then got tired of it. Sad me. Obviously, this is no big deal but if I just type “ok” for a response it would change it to “okay”. I eventually had to look on the Internet to find out where and why this was changed. It's a waste of time.
I should've been able to say “Hey Siri don’t change ok when I'm typing.” or something of that nature. It's Apple's problem NOT mine as a user. You should also be able to ask it why it was doing that. Getting answers to what’s possible is very useful. The second a person leaves the phone to go to the web/webpage to “figure it out” the phone wins and the user loses. I recognize it is hard work for Apple but they have plenty of people and money to make this phone easier for everybody. They just have to be religious about voice solving almost everything. I’m quite serious about that. iOS UI navigation while beautiful, is obsolete really.
So, Apple please make sure voice can do everything in settings!