how do you text fractions with an iphone?

You could just use the number and the forward slash. If you want to format the fraction the way it is in your post, you could add a text replacement.

To create a text replacement:

Copy the fraction on your iPhone the way you want it formatted. Then go to Settings> General> Keyboard> Text Replacement> Add> Put the correctly formated fraction in phrase ex: ¼ > put the unformatted fraction in the shortcut ex: 1/4. After doing that when you type 1/4 it will automatically change into ¼.

-AJ

Unless you want to use a text replacement for every possible fraction, which be impossible, what you typed in your post works fine. It’s just a couple of key strokes rather than using built in keys......1/2, 2/3, 3/4, 13/32, etc, etc.