The pi symbol isn't readily available on your iPhone's default keyboard, but there are tricks you can do to change that and have it available whenever you need.

Access special symbols You may have noticed that if you press and hold on a letter in your iPhone keyboard, all the different variations of how that letter is used in different languages pops up.

To type Does not equal (≠) sign on keyboard in any and all text-editors, you can use any of the methods mentioned in this article.