Missing “Additional CSS” option in Customizer

Since upgrading to WordPress 5.7, the “Additional CSS” option seems to be missing in Customizer at Appearance -> Customize on various WordPress websites I look after with various different themes including Astra and GeneratePress.

I have updated the plugins and themes to the latest versions but this has not helped.

I realise I can add custom CSS via a plugin and possibly via a child theme but what happened to this option?