In reply to Bastian.

Yes you can use it in classic themes.

It depends on how you define a block theme.
For me, a block theme is a theme built entirely of blocks. All the markup for the webpage comes from blocks.
Adding the theme.json will not change the markup of the theme: only the markup of the group block.

What it does, is give the theme author a configuration file for enabling and disabling supported block controls like gradients and drop cap.

It is a configuration file for colors, default spacing and width for the body element and for blocks.

It generates CSS custom properties that theme authors can use for HTML elements that are not blocks, but they have to apply that CSS manually.