Is there an iOS 13 patch to fix the WebKit exploit fixed in 14.4.2?
Apple only provide security updates for older iOS versions for devices that cannot be updated to the current iOS.
If your device is capable of running the current iOS, then your security update is in that latest update. No device is 'stuck' at iOS 13, so there will be no fix for that. The only way to get the security fix is to update to iOS 14.4.2
The last generation of devices, 'stuck' on iOS 12, received that security update in 12.5.2 yesterday.