Since upgrading to WordPress 5.7.1, 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 have tried temporarily changing to a default theme (Twenty Twenty-One) but this does not help either.
I realise I can add custom CSS via a plugin and possibly via a child theme but what happened to this option?
Update: This option appears as expected in a fresh install of WordPress 5.7.1 with the default Twenty Twenty-One theme.