So Siri was missing from my iPad Air 2 and having searched and reviewed a number of google results I still could not find and answer. I was surprised by the number of people experiencing the same problem. Basically Siri was appearing in settings and although listed in restricted apps, that was not the problem.
If you have a newer version iPad that should have Siri and you can't find it even after a reset and backup, the only other explanation is that the iPad is using a restricted ecosystem. For example, a profile set up by your work which restricts Siri. As soon as I removed the work profile, Siri was back again.