Gutenberg block JavaScript localisation on Polyglots does not work

The first argument $handle of wp_set_script_translations() expects the script handle the text domain will be attached to. In your case it’s simpletoc and not simpletoc-js.
The correct call would be:

wp_set_script_translations( 'simpletoc', 'simpletoc' );

Another note: wp_set_script_translations() is available since WordPress 5.0 thus the function_exists() check isn’t necessary.

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