This is a free plugin that reverts WordPress’ Customize screen back to how it was before the Site Editor took over.
It got accepted to the WordPress plugin directory yesterday.
It is effectively only one line of code, as it is simply turning the customize feature back on.
I have already used it on several sites. Mostly for direct access to the site icon uploader and the additional CSS category.
If it comes in handy, please give it a nice review.