I tried to open the file in a new project, but this didn't do anything. When I right click the file and 'open as,' I don't see the option of interface builder. The drop down menu in the identity pane also has no interface builder option.
My main.storyboard's 'default' identity is as xml source code
Using release, non-beta, Xcode 12.x?
What version Xcode was that storyboard originally created with?
What happens if you create a new project and then select/rt. click a .storyboard file...do you see all options then?
Did you use the Product menu to clean build folder?
Did Xcode ask to fix anything in the legacy project?
What version Xcode was that storyboard originally created with?
What happens if you create a new project and then select/rt. click a .storyboard file...do you see all options then?
Did you use the Product menu to clean build folder?
Did Xcode ask to fix anything in the legacy project?
What are all the options you see in the Open as menu ?
Could be your file is corrupted and not recognised as storyboard
Check XML file starts like this (specially document Type), here for XCode11.3:
<?xml version="1.0" encoding="UTF-8"?>
<document type="com.apple.InterfaceBuilder3.CocoaTouch.Storyboard.XIB" version="3.0" toolsVersion="15705" targetRuntime="iOS.CocoaTouch" propertyAccessControl="none" useAutolayout="YES" useTraitCollections="YES" useSafeAreas="YES" colorMatched="YES" initialViewController="BYZ-38-t0r">
<device id="retina6_5" orientation="portrait" appearance="light"/>
<dependencies>
<deployment identifier="iOS"/>
<plugIn identifier="com.apple.InterfaceBuilder.IBCocoaTouchPlugin" version="15706"/>
<capability name="Safe area layout guides" minToolsVersion="9.0"/>
<capability name="documents saved in the Xcode 8 format" minToolsVersion="8.0"/>
</dependencies>
<scenes>
Could be your file is corrupted and not recognised as storyboard
Check XML file starts like this (specially document Type), here for XCode11.3:
<?xml version="1.0" encoding="UTF-8"?>
<document type="com.apple.InterfaceBuilder3.CocoaTouch.Storyboard.XIB" version="3.0" toolsVersion="15705" targetRuntime="iOS.CocoaTouch" propertyAccessControl="none" useAutolayout="YES" useTraitCollections="YES" useSafeAreas="YES" colorMatched="YES" initialViewController="BYZ-38-t0r">
<device id="retina6_5" orientation="portrait" appearance="light"/>
<dependencies>
<deployment identifier="iOS"/>
<plugIn identifier="com.apple.InterfaceBuilder.IBCocoaTouchPlugin" version="15706"/>
<capability name="Safe area layout guides" minToolsVersion="9.0"/>
<capability name="documents saved in the Xcode 8 format" minToolsVersion="8.0"/>
</dependencies>
<scenes>