Can't type 'it' or 'is' on iPhone
It may not work for everyone, but have a try:

Reset 'Keyboard Dictionary'. Go to Settings -&gt; General -&gt; Reset -&gt; Reset Keyboard Dictionary. Then reboot the device and see if it worked.

Another option is to go to Settings -&gt; General -&gt; Keyboard -&gt; Text Replacement and enter 'it' as both text and replacement. Then repeat the process for the 'is'. Again, may not work for everyone.

If neither option helped, wait for the official patch from Apple.