Is there a way to turn off iOS 9 update reminders?

The answers to this question imply that the reminders do not recur... but that's for versions prior to iOS 9.

Much like with newer versions of OSX, iOS 9 will remind you every other day or so to update giving only the options to "update now", "remind me later", "view details". The third option is the only way to avoid "scheduling" a reminder later and it's my go-to option in OSX to shut the reminders off for a bit... but I don't want to stop in the middle of what I'm doing to switch over to the Settings app and then go right back to the app I was actually using.

This is very annoying to me, particularly as I intentionally skip updates. Is there a way to turn this "feature" off?

This is kind of a yes/no question. No, most people can't or won't be bothered to turn them off. Yes - you can turn off all network access or block the update servers.

The iOS 9 itself does not have a toggle you can flip and the software is designed to let you know the update is available. I'll leave my opinion aside whether Apple designed this correctly, but from a security standpoint. If you are on iOS 9 - upgrading to patch levels doesn't break existing apps and solves known security, performance, energy efficiency and bug issues, so declining a 9.0.x or 9.x update is far harder to justify than just not upgrading to 9.0 in the first place.

The best technical way to block iOS 9 updates is hosted on JAMF Nation discussions:

• https://jamfnation.jamfsoftware.com/discussion.html?id=16955

This involves blocking two URLs on your firewall (could be useful if you're using a Wi-Fi only iPad at home):

appldnld.apple.com
mesu.apple.com

You'll want to have control over your network, have an MDM and the energy to configure your devices to avoid updates. Basically, preventing this is a short term battle and I know it makes sense to try it from time to time or if your organization needs to buy time - but it's a pain since the design is to push updates on each device that connects to the internet.

TLDR - Go to General > Storage & iCloud Usage > Manage Storage (under Storage) > Select the update item (should have the settings icon and named iOS 9.x.x) > Delete Update

There is a much easier solution to the nagging reminders explained in the following link.

https://discussions.apple.com/message/29004014#29004014

Yes there is a way to disable it. First let me explain why you are getting that notification. Even if you have automatic updates turned off, the new IOS update still downloads to your phone and iPad. It is already in your phone and iPad right now waiting for you to completely install it. It is not installed but it is sitting there in your phone and iPad waiting for you to click on it to install. So what you must do is the following:

Click on the Setting icon. Then click on General. click on Storage & iCloud usage. It might just say Storage on your device. When the next page pops up you will see at the top the word Storage. Under it you will see Manage Storage. Click on it and wait till the list is fully populated. Depending on how many apps you have this can take a couple of seconds to about half a minute. Look through this list till you see the new iOS update. Delete it. Then your pop up notice should go away. Don't worry about loosing the update. You can always get it later in the future. If in the future you get the pop up again, then that means Apple has again downloaded it to your phone or iPad without your permission. Simply do the above all over again to delete it and get rid of the new pop up. This is the only way to get rid of the pop up without installing it.

There's no other way around. The above feature is hard-coded in to the existing iOS. The only solution is:

Add the following entry in your router to block the DNS:

mesu.apple.com,     23.73.181.0/24

Or if you have Raspberry Pi functioning as router, add the following in iptables:

sudo iptables -A FORWARD -i eth1 -d 23.73.181.0/24 -j DROP #mesu.apple.com

Although I am using Apple's product, there is never any resason for my access any of hosts owned by Apple, Inc. So I am blocking all Apple hosts:

sudo iptables -A FORWARD -i eth1 -d 17.0.0.0/8 -j DROP #apple.com

A much simpler way is to use existing ad blocking apps to block your iPhone/iPad from checking Apple's server - specially mesu.apple.com and applednld.apple.com. These apps filters traffic coming in to you iPhone via a dummy VPN or a proxy auto-config file.

I personally used Weblock on my iPad. There are other apps such as, Adblock or Adguard Pro, that will do the trick too. Note that Safari content blockers won't work.

The overall idea is similar to the other answers here, but since it is not applied on your router, only your iPhone is affected.