Not able to select custom fonts in Xcode 11 beta?

xib and storyboard files are just simple, plain XML files. As with most XML files they're relatively easy to read. Setting a font for a UILabel or UITextField, is just as easy as adding an xml-attribute. In this case a <fontDescription>

In the Xcode navigator, navigate to the xib or storyboard file and choose "Open As->Source Code". In the example you can see a UITextField:

TextField without Font-Attribute

<textField opaque="NO" contentMode="scaleToFill" contentHorizontalAlignment="left" contentVerticalAlignment="center" placeholder="Ihr Vorname" minimumFontSize="17" translatesAutoresizingMaskIntoConstraints="NO" id="RLF-J9-mQz">
    <rect key="frame" x="215.66666666666663" y="16" width="231.33333333333337" height="14"/>
    <textInputTraits key="textInputTraits" autocapitalizationType="words"/>
</textField>

TextField with Font-Attribute

<textField opaque="NO" contentMode="scaleToFill" contentHorizontalAlignment="left" contentVerticalAlignment="center" placeholder="Ihr Vorname" minimumFontSize="17" translatesAutoresizingMaskIntoConstraints="NO" id="RLF-J9-mQz">
    <rect key="frame" x="215.66666666666663" y="16" width="231.33333333333337" height="14"/>
    <fontDescription key="fontDescription" name="SourceSansPro-Light" family="Source Sans Pro" pointSize="17"/>
    <textInputTraits key="textInputTraits" autocapitalizationType="words"/>
</textField>

Save the file an open it again in IB and voilà!