iPhone touch screen malfunction

No. There's really no way to downgrade.

Not saying you did so, but there are some reports that older iPhones with aftermarket replacement screen assemblies are now unresponsive after updating to 11.3.1. With 11.3 it was the iPhone 8/8+. Even then some people were complaining that completely original phones that had never been serviced were unresponsive.

I'd recommend taking it to an Apple Store if you're near one.

Try restoring your iPhone using iTunes following these directions and see if that restores functionality:

If you can‘t update or restore your iPhone, iPad, or iPod touch

Note that your touch screen need not be working to use this method.