@BlabBen - Unfortunately a lot of online ad spend is still geared around Flash, as tablet/mobile audiences still only make a relatively small percentage for many businesses. Ads will be served with a backup image/gif so those users should still see something.
irishgrizzly - It's a PITA, I still have a copy of CS6 because of this. Many Ad servers, networks and publication sites still require Flash files as low as FP 9/10 which CC won't export. Whilst Adobe's decision to not support older formats is better in the long run, a lot of the industry hasn't caught up yet.
Double click do have a repository of Flash templates they've put together, you can access it here:
https://doubleclick-support.appspot.com/repository
Change the filter to AS3/clickTag and it should give you some sample files which are compatible with DC's platform.
There's another example of the code here:
https://support.google.com/dfp_premium/answer/79268?hl=en but as the article mentions there are instances where AS3 clickTag code doesn't seem to fire correctly.
If your ad is being served through Double Click - then you should get it validated fine if you use their example code above.