You can probably find some apps that would offer that kind of ability within them so that you can then copy and paste or share the text into some other app. Or potentially some third party keyboard that might offer that.
That said, I don't really think that T9 is faster for typing purposes, perhaps for some short text only having one hand free type of circumstances, but even with that something like swiping/gliding could likely be faster.
If there's a third party keyboard that offers that, you can install that keyboard and use it. The stock iOS keyboard that Apple offers doesn't have T9 (or swiping/gliding).
There is a third party keyboard (it's called Type Nine), yet sometimes it gets slow and its predictions are not as accurate as those from my very old Nokia phones. I used it for a little while and then went back to the standard iOS keyboard.