For me, this is utterly broken and unusable!

None of my custom, self-coded widgets work, and I am confident that they are all properly coded with the modern API calls, so they should.

I understand the need to modernise, but it shouldn't break backwards-compatibility. These block widgets should honour the CSS classes that old widgets use so that themes don't instantly break.

Like the previous commenter, remove_theme_support( 'widgets-block-editor' ); does not restore the widget editor to the customiser.

There is no way that this is anywhere near being ready for inclusion in Core.

So, for now, I have had to deactivate the Gutenberg plugin.